verb 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 failure or negative consequence.
You must arrive in class on time.
verb To do with certainty; indicates that the speaker is certain that the subject will have executed the predicate.
If it has rained all day, it must be very wet outside.
verb Used to indicate that something is very likely, probable, or certain to be true.
The children must be asleep by now.
noun Something that is mandatory, required or recommended.
If you're trekking all day, a map is a must.
noun The property of being stale or musty.
noun Something that exhibits the property of being stale or musty.
noun Fruit juice that will ferment or has fermented, usually from grapes (in winemaking) or apples (in cidermaking).
The guide then explained that the must drains from the cider press and runs into the vats where it will ferment.