1to behave as if one is an adult; to put on airs of maturity (often overstating one's maturity).
彼はまだ高校生なのに、大人ぶって喫茶店でタバコの話をしていた。
Even though he's still a high school student, he was acting like an adult and talking about cigarettes in the café.
子どもの前で大人ぶる必要はないよ。素直に振る舞えばいい。
You don't need to pretend to be grown-up in front of children. Just behave honestly.
あの人は偉そうに専門用語を並べて大人ぶっているように見える。
That person seems to be putting on airs by stringing together technical terms.