must

Etymology 1

From Middle English moste ("must", literally, "had to", the past tense of Middle English moten (“to have to”)), from Old English mōste (“had to”), 1st & 3rd person singular past tense of mōtan (“to be allowed, be able to, have the opportunity to, be compelled to, must, may”), from Proto-Germanic *mōtaną. Cognate with Dutch moest (“had to”), German musste (“had to”), Swedish måste (“must, have to, be obliged to”). More at mote.

verb

1. (modal auxiliary, defective) To do as a requirement; indicates that the sentence subject is required as an imperative or directive to execute the sentence predicate, with failure to do so resulting in a failure or negative consequence.
   You must arrive in class on time. (the requirement is an imperative)
   This door handle must be rotated fully. (the requirement is a directive, necessary to operate the handle)
2. (modal auxiliary, defective) To do with certainty; indicates that the speaker is certain that the subject will have executed the predicate.
   If it has rained all day, it must be very wet outside.
   You picked one of two, and it wasn't the first: it must have been the second.
3. (modal auxiliary, defective) Used to indicate that something is very likely, probable, or certain to be true.
   The children must be asleep by now.

noun

1. Something that is mandatory or required.
   If you're trekking all day, a map is a must.

Etymology 2

From Middle English must, from Old English must and Old French must, most, both from Latin mustum.

noun

1. The property of being stale or musty.
2. Something that exhibits the property of being stale or musty.
3. Fruit juice that will ferment or has fermented, usually from grapes.
   No sweet grape lies hidden here in the shade of its vine-leaves, No fermenting must fills and o'erflows the deep vats. c. 1874, Henry Wadsworth Longfellow, Ovid in Exile

verb

1. (transitive) To make musty.
2. (intransitive) To become musty.

Etymology 3

From Persian مست (mast, “drunk, inebriated”), from Middle Persian 𐭬𐭮𐭲 (mast).

noun

1. Alternative form of musth
   It was not, of course, a wild elephant, but a tame one which had gone ‘must’. 1936, George Orwell, Shooting an Elephant, an essay in the magazine New Writing