backside

Short definition from Wiktionary

Noun

1. 1
   The back side of anything, the part opposite its front, particularly
2. 2
   The back side of anything, the part opposite its front: The back side of an estate: the backyard and outbuildings behind a main house, especially (UK dialect, euphemistic) an outhouse.
3. 3
   The back side of anything, the part opposite its front: A person's buttocks.
4. 4
   The back side of anything, the part opposite its front: The back side of a page: a verso.
5. 5
   The reverse or opposite of anything.
6. 6
   Used other than figuratively or idiomatically: see back, side.

Adjective

1. 1
   Approaching an obstacle backward.