Real Dictionary

must
modal verb: You use must to indicate that you think it is very important or necessary for something to happen. You use must not or mustn't to indicate that you think it is very important or necessary for something not to happen.

must in American English 1
used to express compulsion, obligation, requirement, or necessity
adjective: necessary; vital
auxiliary verb: to be obliged or bound to by an imperative requirement
intransitive verb: to be obliged; be compelled
noun: something necessary, vital, or required

must in American English 2
a state of frenzy in animals, esp. in the male elephant, usually associated with sexual heat
in must
noun: new wine; the unfermented juice as pressed from the grape or other fruit

must in American English 3
the juice pressed from grapes or other fruit before it has fermented; new wine
noun: mold; moldiness; mustiness

must in American English 4
a musty quality or state; mustiness
noun

must in American English 5
noun: musk, esp. a powder made from musk
transitive verb: to powder (the hair)

must in British English 1
noun: an essential or necessary thing
verb: used as an auxiliary to express obligation or compulsion: you must pay your dues. In this sense, must does not form a negative. If used with a negative infinitive it indicates obligatory prohibition

must in British English 2
noun: mustiness or mould

must in British English 3
noun: the newly pressed juice of grapes or other fruit ready for fermentation

must in British English 4
noun

must-
combining form: Must- is added to verbs such as 'see', 'have', or ' read' to form adjectives and nouns which describe things that you think people should see, have, or read. For example, a must-have is something which you think people should get, and a must-win game is o

must- in British English
combining form: indicating that something is highly recommended or desirable