MUST

(modal auxiliary, defective) to do with certainty; indicates that the speaker is certain that the subject will have executed the predicate.

Example: "If it has rained all day, it must be very wet outside."

(modal auxiliary, defective) to do as a requirement; indicates that the sentence subject is required as an imperative or directive to execute the sentence predicate, with failure to do so resulting in a negative consequence.

Example: "This door handle must be rotated fully."

(modal auxiliary, defective) said about something that is very likely, probable, or certain to be true

Example: "The children must be asleep by now."

To make musty.

To become musty.