grown-up; full-fledged adult; sensible adult
Used to emphasize that someone is a mature adult, often in contexts where childish or irresponsible behavior is surprising or inappropriate. Can refer to either gender.
It's shameful for a grown adult to cry over something like that.
A full-grown adult got lost and asked a child for directions.
The prefix 大の (dai no) intensifies the following noun, here 大人 (otona, 'adult'), yielding the meaning 'a full-fledged adult'.