デコピン

noun

finger flick to the forehead

A playful or teasing gesture where one flicks someone's forehead with a finger, often producing a light popping sound. Common among children or in casual settings.

• 弟おとうとにデコピンしたら、泣ないちゃった。

  When I flicked my little brother's forehead, he started crying.

• デコピンって結けっ構こう痛いたいよね。

  A forehead flick actually hurts quite a bit, doesn't it?