/ Dictionary / Index K keystone joist: Meaning and Definition of Find definitions for: key'stone joist" Pronunciation: [key] a reinforced-concrete joist with sloping sides and the top wider than the bottom. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Keystone comedy Keystone Kop Related Content Daily Word Quiz: portent Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language