/ Dictionary / Index M mustard oil: Meaning and Definition of Find definitions for: mus'tard oil" Pronunciation: [key] oil expressed from the seed of mustard, used chiefly in making soap. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mustard gas mustard plaster Related Content Daily Word Quiz: winsome Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language