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The word "Chronic" means "long lasting", or "occurring over an extended period of time". A chronic illness one that you will have for a long time (if not for your entire life), or take a long time to recover from. A short-lived, sharp/intense/severe illness would instead be referred to (in formal language) as "Acute"

However, in informal usage, "Chronic" is frequently used to mean "severe" - such as a sudden painful migraine being referred to as a "chronic headache" even when relatively short-lived.

The word is also used in informal British English to mean 'bad, intense, severe, objectionable'
_{"Fowler's Dictionary of Modern English Usage" (1926) Henry Watson Fowler. Edited by Jeremy Butterfield in 2015}

Certainly in Medical circles, you would never expect to see "Chronic" used in this way, even though it is a "new definition" for the word. However, someone recently asserted thusly to me (regarding the definition of a different word):

"There has to be some period of time after which the new lexical definition of a word can be accepted"

There are several questions this raises (I'm sure that Law.SE could find examples of when this would "invalidate" various Legal documents or rulings!), but the main one I was wondering is this:

What is the first record we have of "Chronic" being used in place of "Acute", its antonym?

There are other word-pairs in a similar situation, such as "Literally"/"Metaphorically", but this seemed an example with significant impact if the colloquial usage were to replace the official one

In medical use, the antonymy of the pair 'acute-chronic' is relational along a temporal axis. That is, 'acute' and 'chronic' contrast with each other in terms of duration or recency: 'acute' may denote something (a disease, disorder, complaint, case, etc.) brief or recent, and yet severe or mild; 'chronic' may denote something long-lasting and severe or mild. Insofar as the medical senses of the terms are inately opposed, it is with respect to time rather than intensity.

Complicating that relational antonymy, 'acute' has been used in medicine to mean "severe; critical" (OED: acute, adj. and n., sense A1a). That meaning does not oppose the meaning of 'chronic'. The use, however, does occur in contexts very similar to uses with the sense "of rapid onset and short duration; of recent or sudden onset" (op. cit.).

Later use of 'chronic' with the colloquial sense of "bad, intense, severe, objectionable" (OED: chronic, adj., sense 3) emerged from use with the sense of "continuous, constant" (op. cit.).

OED attests the sense of "continuous, constant" from 1861 (Considerations on representative government, John Stuart Mill, p. 30):

If, instead of struggling for the favours of the chief ruler, these selfish and sordid factions struggled for the chief place itself, they would certainly, as in Spanish America, keep the country in a state of chronic revolution and civil war.

Mill's is unlikely to be the first use in print of 'chronic' with the meaning "continuous, constant". The same phrase Mill used, 'chronic revolution', for example, appears in an 1847 article, "The Aristocracy of Names" (The Living Age, v. 15, p. 57), and again in an 1849 book, The Temporal Benefits of Christianity (Robert Blakey, p. 293).

The colloquial use of 'chronic' to mean "bad, intense, severe, objectionable" must have been well-established before J. Redding Ware's observations regarding the "passing" (that is, transitory) use of 'chronic' and the slang phrase 'chronic rot', in Passing English of the Victorian Era (1909, pp. 74-5):

 Chronic (M. Class [middle class], 1896). Ceaseless, persistent. 'Oh! Joe's chronic.' 'Charley's Aunt's chronic' — said of a piece that ran perpetually.

 Chronic Rot (Peoples'). Despairingly bad...'Oh, that theatre's chronic' — means that never is a good piece seen there. These two words intensify each other. 'Jack's swears to swear off' (drink) 'is chronic rot.'

The 'chronic' in 'chronic rot' intensifies 'rot'; by the 1890s, 'chronic' has acquired and is being used with the sense of "intense, severe".