noun
adultness; maturity
Refers to the quality of being grown-up, responsible, or emotionally mature. Often used in negative expressions like 大人気ない (childish, immature).
彼の行動は大人気ない。
His behavior is childish.
大人気のある対応を心がけたい。
I want to try to act in a mature manner.
Compound of 大人 (おとな, 'adult') and 気 (け, 'sign, feeling, spirit'). The 気 here indicates a quality or disposition.