尻しりっぺた

noun

buttocks; backside; bottom

Colloquial and slightly childish or informal term for the buttocks. Often used in casual or affectionate contexts.

• 赤あかちゃんの尻しりっぺたはぷにぷにしてる。

  The baby's bottom is soft and squishy.

• 転ころんで尻しりっぺたを打うった。

  I fell and hit my backside.