Definition of must:
(modal auxiliary verb, defective) The speaker—but not necessarily the subject of the sentence—is certain
that the sentence predicate
will be executed; (modal auxiliary verb, defective) The execution of the sentence predicate
. (Must usually implies that a negative consequence would otherwise result.)
"if it rained all day, it must be very wet outside."
something that exhibits the property of being stale or musty; fruit juice that will ferment or has fermented, usually grapes
"if you'll be out all day, sunscreen is a must."