I. ˌgrown-ˈup 1 BrE AmE adjective
1 . fully developed as an adult:
Before you know it, the children will be grown-up and leaving home.
I’ve got two grown-up sons.
2 . behaving in a responsible way, like an adult SYN mature ⇨ childish :
I expect more grown-up behaviour of you.
II. ˈgrown-up 2 BrE AmE noun [countable]
an adult – used by or to children:
If you’re frightened, tell one of the grown-ups.