/ Dictionary / Index M mustn't: Meaning and Definition of Find definitions for: must•n't Pronunciation: (mus'unt), [key] contraction of must not. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. musth must-see Related Content Daily Word Quiz: cautious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language