Must1  —v.aux. (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). B in ironic questions (must you slam the door?). 2 be certainly (you must be her sister). 3 ought to (must see what can be done). 4 expressing insistence (must ask you to leave). 5 (foll. By not + infin.) A not be permitted to, be forbidden to (must not smoke). B ought not; need not (mustn't think he's angry; must not worry). C expressing insistence that something should not be done (they must not be told). —n. Colloq. Thing that should not be missed (this exhibition is a must).  i must say often iron. I cannot refrain from saying (i must say he tries hard; a fine way to behave, i must say). Must needs see *needs. [old english]