b: have permission to <you may go now>: be free to <a rug on which children may sprawl — C. E. Silberman> —used nearly interchangeably with can
c —used to indicate possibility or probability <you may be right><things you may need> ; sometimes used interchangeably with can<one of those slipups that may happen from time to time — Jessica Mitford> ; sometimes used where might would be expected <you may think from a little distance that the country was solid woods — Robert Frost>
—used in auxiliary function to express a wish or desire especially in prayer, imprecation, or benediction <may the best man win>
—used in auxiliary function expressing purpose or expectation <I laugh that I may not weep> or contingency <she'll do her duty come what may> or concession <he may be slow but he is thorough> or choice <the angler may catch them with a dip net, or he may cast a large, bare treble hook — Nelson Bryant>
:shall, must —used in law where the sense, purpose, or policy requires this interpretation
Usage Discussion of MAY
Can and may are most frequently interchangeable in senses denoting possibility; because the possibility of one's doing something may depend on another's acquiescence, they have also become interchangeable in the sense denoting permission. The use of can to ask or grant permission has been common since the 19th century and is well established, although some commentators feel may is more appropriate in formal contexts. May is relatively rare in negative constructions (mayn't is not common); cannot and can't are usual in such contexts.
Middle English (1st & 3d singular present indicative), from Old English mæg; akin to Old High German mag (1st & 3d singular present indicative) have power, am able (infinitive magan), and perhaps to Greek mēchos means, expedient