鬼おにの首くびを取とったよう

expression, na-adjective

triumphant over a small success; boastful about a minor achievement

Describes someone acting as if they have achieved something great when the accomplishment is actually mediocre. Often used sarcastically or critically.

• 彼かれは簡かん単たんな問もん題だいが解とけただけで、鬼おにの首くびを取とったように喜よろこんでいる。

  He's acting as if he's grabbed a demon by the neck just because he solved a simple problem.

• そんな小ちいさなことで鬼おにの首くびを取とったような顔かおをするな。

  Don't look so triumphant over such a small thing.