must
verb
°(modal _ auxiliary) To do with certainty; .
"If it rained all day, it must be very wet outside."
°(modal _ auxiliary) To do as a requirement; .
"You must arrive in class on time. — the requirement is an imperative"
noun
°Something that is mandatory or required.
"If you'll be out all day, sunscreen is a must."
°Something that exhibits the property of being stale or musty.