also: よいおとな
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
Don't cry over something like that—you're a grown adult.
You're old enough to know better, so make your own decisions.
Most common spelling; いい is the everyday form.
More formal or written variant; 良い is read よい here.
Phrase composed of いい (good, proper) and 大人 (adult). The combination literally means 'a proper adult', but is often used ironically.