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