godan verb (-ru)
act like an adult; pretend to be grown-up
Often used for children or young people trying to appear mature, sometimes with a slightly negative or mocking tone.
弟は大人ぶって難しい本を読んでいる。
My younger brother is reading a difficult book, acting like an adult.
彼女はまだ高校生なのに、大人
Even though she's still a high school student, she tried to drink wine, pretending to be an adult.