いい大人おとな

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

• いい大人おとながそんなことで泣なくなよ。

  Don't cry over something like that—you're a grown adult.

• いい大人おとななんだから、自じ分ぶんのことは自じ分ぶんで決きめなさい。

  You're old enough to know better, so make your own decisions.