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must (1)  /mst/  ; weak form /mst/ auxiliary verb used with other verbs without the infinitive "to" 1 expressing obligation: I must go to work tomorrow. 2 expressing the likelihood of s.t.: Her car is gone, so she must have left already. See: modal auxiliary.
n. s.t. necessary, a necessity: A warm winter coat is a must in Toronto.

Thesaurus: must (1) auxiliary verb 1 to have to, ought to 2 probably, maybe.

Usage Note: Compare ought to, would, should, and might. The past tense of must is usu. had to: I had to go to work yesterday. Use must have & past part. to indicate that you are quite sure s.t. happened: If the books are gone, Diane must have taken them.