/ Dictionary / Index M Middlesex: Meaning and Definition of Find definitions for: Mid•dle•sex Pronunciation: (mid'l-seks"), [key] — n. a former county in SE England, now part of Greater London. a borough in central New Jersey. 13,480. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. middle school middle-sized Related Content Daily Word Quiz: gregarious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language