Definition of must:

part of speech: noun

The unfermented juice of ripe grapes; new wine.

part of speech: verb

An auxiliary verb which implies necessity on obligation.

Usage examples:

• Don't you think they must be-? Virginia Woolf in "Night and Day".

• Yes, I must speak. Marie Belloc Lowndes in "Jane Oglander".

• We must have done it. Charles Weathers Bump in "The Mermaid of Druid Lake and Other Stories".

• I have something to show you, but I must see mother for a minute first. Pauline Lester in "Marjorie Dean High School Freshman".