backside
UK: ˈbæk.saɪd | US: ˈbæk.saɪd
n. the rear part of something, especially the human body (buttocks)
n. the reverse or less visible side of an object
The word "backside" is a straightforward compound of "back" (Old English bæc, meaning "rear") and "side" (Old English sīde, meaning "flank" or "surface"). It originally referred neutrally to the rear surface of an object or body, but over time, it colloquially narrowed to specifically mean "buttocks" in modern English. The logic follows the physical orientation of the body, where the "back side" is the opposite of the front.
She slipped and landed hard on her backside.
The backside of the painting was signed by the artist.
He leaned against the backside of the building to avoid the wind.
The horse had a distinctive mark on its backside.
Check the backside of the document for additional instructions.