'grownup' definitions:

Definition of 'grownup'

From: WordNet
adjective
(of animals) fully developed; "an adult animal"; "a grown woman" [syn: adult, big, full-grown, fully grown, grown, grownup]
noun
A fully developed person from maturity onward [syn: adult, grownup] [ant: juvenile, juvenile person]

Definition of 'grownup'

From: GCIDE
  • grownup \grownup\ adj. fully developed; adult; mature; -- of people and animals; as, Act like a grownup!.
  • Syn: adult, big, full-grown, fully grown, grown. [WordNet 1.5]