/ Dictionary / Index B book gill: Meaning and Definition of Find definitions for: book' gill" Pronunciation: [key] the gill of a horseshoe crab, composed of numerous membranous structures arranged like the leaves of a closed book. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. booker bookie Related Content Daily Word Quiz: secular Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language