/ Dictionary / Index U underpart: Meaning and Definition of Find definitions for: un•der•part Pronunciation: (un'dur-pärt"), [key] — n. the lower part or side: The underpart of the plane's fuselage scraped the treetops. an auxiliary or secondary part or role. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: underpart (Thesaurus) underpants underpass Related Content Daily Word Quiz: fabricate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language