/ Dictionary / Index U underactive: Meaning and Definition of Find definitions for: un•der•ac•tive Pronunciation: (un"dur-ak'tiv), [key] — adj. insufficiently active: an underactive thyroid gland. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: underactive (Thesaurus) underact underage 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