Entry 1 sense Webster, 1913 grownup · IPA /ɡrˈoʊnˌʌp/ 01 adj. fully developed; adult; mature; -- of people and animals; as, Act like a grownup!. 1. fully developed; adult; mature; -- of people and animals; as, Act like a grownup!. Syn. adult, big, full-grown, fully grown, grown.