backside

1. The back side of anything, the part opposite its front, particularly
2. The back side of an estate: the backyard and outbuildings behind a main house, especially (UK dialect, euphemistic) an outhouse. · The building's backside faced an alley and was covered in grime and graffiti.
3. A person's buttocks. · Having ridden the horse all day for the first time, I had painful blisters on my backside.
4. The reverse or opposite of anything. (figuratively)
5. Used other than figuratively or idiomatically: see back, side.

الجمع backsides