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adultness; maturity
「大人気」 has another reading with different meanings.
大人気ないことをするな。
Do not do something so childish.
子供の前であんな大人気ない態度を取ってしまった。
I ended up acting so immaturely in front of the children.
彼の大人気なさに皆が呆れた。
Everyone was exasperated by his immaturity.