Meaning of MUST in English

v. & n.

v.aux. (3rd sing. present must; past had to or in indirect speech must) (foll. by infin., or absol.) 1 a be obliged to (you must go to school; must we leave now?; said he must go; I must away). °The negative (i.e. lack of obligation) is expressed by not have to or need not; must not denotes positive forbidding, as in you must not smoke. b in ironic questions (must you slam the door?). 2 be certain to (we must win in the end; you must be her sister; he must be mad; they must have left by now; seemed as if the roof must blow off). 3 ought to (we must see what can be done; it must be said that). 4 expressing insistence (I must ask you to leave). 5 (foll. by not + infin.) a not be permitted to, be forbidden to (you must not smoke). b ought not; need not (you mustn't think he's angry; you must not worry). c expressing insistence that something should not be done (they must not be told). 6 (as past or historic present) expressing the perversity of destiny (what must I do but break my leg).

n. colloq. a thing that cannot or should not be overlooked or missed (if you go to London St Paul's is a must). I must say often iron. I cannot refrain from saying (I must say he made a good attempt; a fine way to behave, I must say). must needs see NEEDS.

[ OE moste past of mot may ]

Concise Oxford English dictionary.      Краткий оксфордский словарь английского языка.