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As this is medieval times, I think he disguises himself as a leper. He has a long shabby hooded cloak. On his feet he has boots glued on and gloves glued onto his hands.

He walks through the street tinkling a bell and shouting, "Unclean! Unclean!". His hood is drawn over his face with just a gap to see out of. His skull is painted black so there is nothing to see inside. As an added touch he can wear an unkempt wig (see Leviticus below).

Most people will want to avoid him and will not get close enough to look inside his hood even if they wanted to.

Biblical reference

Leviticus 13:45

"Anyone with such a defiling disease must wear torn clothes, let their
hair be unkempt, cover the lower part of their face and cry out,
'Unclean! Unclean!'
https://biblehub.com/leviticus/13-45.htm

Religious robes

Loose religious robes with a hood and perhaps a mask would do the trick. In a fantasy setting with many gods, there may well be a religion with suitably masked and robed followers. Below, a bunch of Spanish priests in an Easter procession. Penitent monks also typically wore hoods and walked with heads bent, making their faces hard to see.

Mummer

Masked entertainers or mummers weren't uncommon in Medieval times. During festivals, mummers probably weren't an unusual sight in city streets.

Plague Doctor
Especially during times of plague, doctors went about with snouted face masks holding herbs believed to cleanse the air. If your world has something similar, that should also work.

Pretend to be a marionette controlled by a ventriloquist

Like Weekend at Bernie's but in reverse: appear to be the dead skeleton that you are, but pretend there is a living person controlling you and speaking for you.

There's a couple ways to accomplish this the easiest is to set up a booth at the fair and tie some strings to your limbs. Loop the strings over a stick above you, and have the other ends go behind a curtain. Attach weights to the other end of the strings so they'll maintain tension and seem like someone is pulling on them. You won't be able to move about (unless you put this setup on a cart) but people will be enticed by the spectacle and even pay money to come have conversations with you.

This could lead to a great moment where someone insists on seeing the talented puppeteer and pulls back the curtain only to realize there is no one there.

Another bonus is that you don't have to assume any fake persona. No one will catch onto you when you accidentally reveal how ignorant you are about humans and normal life, because they'll assume that's part of the act. When people ask you to tell your puppet character's story you can tell your actual backstory.

Concealing clothing has been suggested and could work, but I'd add on a layer of leather/dirt. People might look under your clothes after all or bump into you and you don't want to be obviously a skeleton under them.

So take leather and cover all your bones in it. Then add a layer of dirt/grime to it. You also want to fill in the leather with some thing soft and squishy, maybe more leather/dead leaves/rags to get you more like a human shape. That way if someone bumps into you or touches you you don't feel like just a skeleton. The dirt is then added to hopefully make people think the leather is your skin and is just dirty.

Even with all of this your face will look weird and dirty at best. Just claim you were wounded as a child and look weird, you could also claim to be sick but that might get you kicked out.

If its medieval time, he can wear full body knight armour. Its not easy to see, that there is skull below the helmet. Probably he can fill the empty places in armor with clothes or rags, where muscles and flesh have to be for humans.

Humans have skeletons inside them, but they are also muscle, tendons, skin, among other things. Lucky for your skeleton, it doesn't need muscles or guts. Also lucky for your skeleton, muscles and guts are easy to duplicate! Let's assume your skeleton has seen people at a distance, enough to know how they act, look, etc.

After some basic observation, move on to some up-close examination of a human, via grave-robbing! A little knife-work will show how humans look under all that skin, and give insight into the next step.

Using leather stuffed with wool or cotton, the skeleton makes muscles, and using strings as tendons, ties them to itself. A little balled up cotton or wool, and he has guts, too. Next, layers of cloth, wrapped around the "muscles" and bones, to simulate fat. Since it will all be under a layer of clothes, the skeleton's work could be fairly clumsy and still pass muster.

The last step is most difficult. Shoes and clothes can hide everything but the skeleton's face and hands; depending on how it dresses, it might even be able to wear gloves. Its head, however, is still obviously a bleached skull. A mask of some sort, even just a handkerchief over the mouth and nose (maybe with some clay to form features), and a hood would work for a little while. The skeleton is ready to go exploring - at least a little bit.

It should start exploring in winter, while everyone is bundled up; using whatever treasure the skeleton has collected, it should go to town, walk in like it owned the place, and pick out a lovely wig. If possible, it should find some fake eyes, and any other disguise-worthy materials, including makeup.

With disguise in hand, the skeleton can make itself appear entirely human, apart from one small detail: its face. This is where its fake eyes come in. Using clay and a little tanned animal skin, make a face - doesn't have to be a good face, just vaguely lifelike. Using the makeup, give the face a horribly scarred appearance, like a burn victim. Finally, don a simple mask.

Now, the skeleton looks and feels human, and can explain why it wears a mask: due to an accident, it suffered terribly burns from fire/acid/lightning/whatever, and is forced to hide its ugly skin under this mask.

The longer it spends with humans, the more it learns, and eventually, it may find someone willing to create a lifelike skin for it. Without very complex mechanical tools, it will never have the muscles to smile, blink, breathe, or wiggle its eyebrows, but it will eventually be able to pass for human. As long as no one looks too hard.

Masquerade

https://en.wikipedia.org/wiki/Masquerade_ball

Those with a lot of wealth in Medieval and Renaissance societies sometimes held masquerade balls. These were parties where all the participants wore masks and costumes, obscuring the identity of the wearer. These kinds of parties have been popularised in fiction due to the novel idea.

Your skeleton may sneak into such a party in masquerade and no one would know he was a skeleton because, of course, he is in a costume, as is everyone else, allowing him to easily blend in. This also allows for your skeleton to interact with a lot of humans freely, asking questions to the many guests at the party.

Entertainment

In a similar sense, your skeleton could wear a costume and play the role of an entertainer, giving a plausible reason as to why they are always in costume. You could also have the skeleton talk to the audience members after a show or performance.

The Man in the Iron Mask

Your skeleton could possibly wear a mask and claim to be a convicted criminal and, as punishment, must wear this mask. Although, whilst this would effectively obscure his identity, i doubt people would be willing to talk with him as why would they want to chat with a possibly dangerous criminal?

The Dark Side of the City

Whilst your average person may be terrified of this reanimated skeleton wandering the streets, i would wager a guild of thieves or assassins would be far more welcoming. Such guilds may even be very willing to enrol a reanimated skeleton into their ranks, such a member could allow for a wide variety of interesting situations. For example, a skeletal thief could hide in a coffin, be placed in a crypt and grave rob said crypt from the inside. An assassin may be able to sneak past a blockade in a coffin to assassinate the king.

Your skeleton would interact with the members of its guild, learning about humanity that way, and could also spy on its mark and learn through their conversations with other people or through letters they write.

A Home for the Blind

Its possible that a skeleton could go unnoticed in a home for people who are blind, either working as a servant for a single wealthy blind person or for a community of blind people, the latter allowing for more social interaction.

This way, your skeleton may not need to physically disguise themselves, only hide the texture of their bones, such as through clothing or some kind of fake skin. Alternatively, you could go with the idea that the blind people know he is a skeleton but simply do not care as he is kind to them. I would imagine a person’s physical appearance would matter far less to someone who is blind, instead judging people more by their personality and their actions.

Just thinking out of the box here:

No Disguise at All

Instead, just tell everyone a witch cursed you to have invisible flesh, but it doesn't affect your bones. Sure, you said "no magic", but this isn't magic. It's telling people that magic was involved, but it's not actual magic. (At least, not any more magic than was required by the necromancer that animated the skeleton to begin with, which we're obviously taking as a given here.) In a world where necromancy is a thing, I'm sure people would have no problem believing that invisibility curses are also a thing (even if that's not actually true), so just run with it. If you're friendly and personable enough, people might look past the fact that you're a walking skeleton. Offer your services, be a good neighbor, and just calmly explain your "curse" whenever asked. You'll get some stares and fearful looks, but just don't act threatening and you can fit right in. Real life burn-victims and people with other visible disfigurements have to deal with this sort of thing all the time.

Here's some options. He could...

• Wear bandages and say he's a burn victim


• Get a job as a clown (interview as burn victim)


• Get a job in a dark restaurant like this one (https://dineblackout.com/)


• Get a job at Disney World dressed as Mickey Mouse


• Buy a computer and make all his friends online


• Build a box around himself and pretend to be an animated machine designed to pass the Turing Test; then sit at a carnival and wait for people to drop money in the coin slot for a conversation


• Buy a lifelike puppet and pretend that it is the human, and he's the puppet


• Go to comicon


• Go to the medieval renaissance fair


• Join the KKK and wear shoulder pads under his robes or something


• Join a dnd club where people dress up as their fave characters


• Hide in the drain under the street and have conversations with kids passing by


• Go find a remote tribe in the jungle where they'll think he's a god, and talk them into interacting normally with him over the course of a few generations


• Go deep sea diving and wear his diving suit everywhere


• Become a death-metal musician, and act like the skeleton thing is just his excellent show makeup


• Hang out near a volcano and wear a heat-suit; interact with geologists.

Hope this helps!

eta: (Read through some other answers and realized some of these were already taken. Didn't mean to copy! Sorry other dudes!)

A skeleton under flapping robes still doesn't look like a person. Make yourself into a scarecrow -- acquire clothing, put on clothing, stuff your clothes with straw, then top it all off with a loose cloak and deep hood.

The way I would see it is simple. Kill a human. We humans have skeletons, do we not? So, the skeleton could kill a human, remove the humans skeleton using clean cuts creating seems along the skin, lie inside of the removed skin (and likely meat, since it would need "padding". Either that, or some other material to act in the same way. Then, basically stitch the cuts back to close the costume.

None of this would require very advanced technology. Just a blade like object and a tool like needle and thread. Assuming the skeleton was intelligent and logical about how it skinned the human, it could do so in such a way that all of the seems would be hidden by clothing.

Grave diggers were usually considered unclean, so people didn't have much to do with them. By day stand still and be a grave decoration, by night dig graves and chit chat with the grave digger.

Be a beggar, in general wear dirty rags and hang out places, perhaps stick a dead rodent in your robes so you smell bad enough for people to not get close.