must

verb

1. (auxiliary, defective, modal, no-past-participle, no-present-participle)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.”

   “This door handle must be rotated fully.”

   “Everybody told me I must stop worrying, but I couldn't, because I felt there must be something wrong.”

   “You really must come and see us soon, pal. - We'll see: I can borrow my dad's car, but I must bring it back before midnight.”

   “The physician said that I must / had to / would have to stop smoking.”

2. (auxiliary, defective, modal, no-past-participle, no-present-participle)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.”

   “Where would you have put the keyring? You couldn’t have lost it again! - I must have, because it's nowhere to be found.”

   “I said it must be pretty late and I really must go.”

   “She's being taken to the police car now , so they must be going to arrest her.”

3. (auxiliary, defective, modal, no-past-participle, no-present-participle)Used to indicate that something is very likely, probable, or certain to be true.

   “The children must be asleep by now.”

   “There's the doorbell. It must be Dad.”

4. (auxiliary, defective, modal, transitive)To make musty.
5. (auxiliary, defective, intransitive, modal)To become musty.

noun

1. (auxiliary, defective, modal)Something that is mandatory, required or recommended.

   “If you're trekking all day, a map is a must.”

   “"There are no musts in my life - I'm free, white, and twenty-one."”

2. (auxiliary, countable, defective, modal, uncountable)The property of being stale or musty.
3. (auxiliary, countable, defective, modal, uncountable)Something that exhibits the property of being stale or musty.
4. (auxiliary, countable, defective, modal, uncountable)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.”

5. (alt-of, alternative, auxiliary, countable, defective, modal, uncountable)Alternative spelling of musth.

   “No animal in the world is so dangerous as an elephant in must.”

   “Early one morning the sub-inspector at a police station the other end of the town rang me up on the 'phone and said that an elephant was ravaging the bazaar. Would I please come and do something about it? […] It was not, of course, a wild elephant, but a tame one which had gone "must". It had been chained up, as tame elephants always are when their attack of "must" is due, but on the previous night it had broken its chain and escaped.”