/ Dictionary / Index M masto-: Meaning and Definition of Find definitions for: masto- Pronunciation: [key] a combining form meaning “breast,” used in the formation of compound words: mastopathy. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mastitis mastocarcinoma Related Content Daily Word Quiz: censure Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language