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why dictionary of american idioms says "of is usually retained before pronouns"

why dictionary of american idioms says "of is usually retained before pronouns" such as order someone off ((of)something) to command someone to get off something.(Of is usually retained before pronouns) The teacher ordered Tom off of his steps

For "retained" read "inserted"

In UK English, the usual expression is

The teacher ordered Tom off his steps.

I suppose the dictionary is telling you that in idiomatic American English, the expression is

The teacher ordered Tom off of his steps.

I suppose "retained" means "not deleted".

I am not sure why it is considered "retained", because I can't think of where the "of" would have been deleted from. I don't consider the UK English version to have lost a word. I suggest you interpret it as "In idiomatic American English, the word of is inserted".

I don't see why the dictionary mentions pronouns. As far as I know, there is no limitation for the "of" to only occur before a pronoun. I think it is just as correct American idiom to say, "The teacher ordered Tom off of Mary's steps". Meanwhile, in the UK it would be "The teacher ordered Tom off Mary's steps".

In the UK inserting the "of" would be considered a very informal or casual usage, rather than standard.

American idiom has "of" inserted after "off" for several verbs

Br E: "Can I bounce an idea off you?" 

    (Tell you an idea, and hear your reaction to it)

Am E: "Can I bounce an idea off of you?"



Br E: "Get off that horse!"

Am E: "Get off of that horse!"

I don't know the origin of this. Perhaps UK English has indeed lost an "of"? Both Englishes say

"Get out of that house"

"Get rid of that horse"