also: よいおとな
expression, noun
grown adult; person old enough to know better
Often used sarcastically to criticize someone for acting immaturely despite being an adult. The phrase implies 'an adult who should know better'.
See also: いい年をして
いい大人がそんなことで泣くなよ。
Don't cry over something like that—you're a grown adult.
いい大人なんだから、自分のことは自分で決めなさい。
You're old enough to know better, so make your own decisions.
Similar sarcastic phrase meaning 'at your age' or 'for someone your age', used to scold childish behavior. いい大人 focuses on the person, while いい年をして focuses on the age.
Phrase composed of いい (good, proper) and 大人 (adult). The combination literally means 'a proper adult', but is often used ironically.