Etymology 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.
1
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.
4 more examples
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.
Arise, and go into the city, and it shall be told thee what thou must do.1611, The Holy Bible, […] (King James Version), London: […] Robert Barker, […], →OCLC, Acts 9:6:
2
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.
2 more examples
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
Defective, Modal, No past participle, No present participle Used to indicate that something is very likely, probable, or certain to be true.
Etymology 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.
1
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."1932, Helen Vinson, I Am a Fugitive from a Chain Gang:
must
noun
Etymology From Middle English must, from Old English must and Old French must, most, both from Latin mustum.
1
Defective, Modal The property of being stale or musty.
2
Defective, Modal Something that exhibits the property of being stale or musty.
3
Defective, Modal 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: