What is the history of "may" being used to mean "must"?

According to (online) Merriam-Webster, "may" has the following two distinct definitions, among others

1 b: have permission to

4: SHALL, MUST —used in law where the sense, purpose, or policy requires this interpretation

Oxford dictionary does not seem to have the definition of may = shall, must.

What is the history of may being used to mean "must"?

Solution 1:

The OED dates this sense of the word back to 1715:

Where a Statute directs the doing of a Thing for the sake of Justice or the publick Good, the Word may is the same as the Word shall; thus 23 H.6. says, the Sheriff may take Bail; this is construed, he shall; for he is compellable so to do.

—Reports of cases adjudg'd in the Court of King's bench

Solution 2:

As a native UK speaker, I'd simply say that Merriam Webster is wrong.

The 1715 example just seems to be noting that when a statute says 'may', that - in the particular circumstances relating to the Sheriff - is the same as 'shall', because "he is compellable so to do".

If he wasn't compellable to do so, 'may' would not be the same as 'shall' or 'must'.

Even legally speaking, if someone tells me I 'may' use their swimming pool, that does not mean I 'must' or 'shall' do so - because I'm not under any obligation to go swimming whenever I can.