backside

noun

1. the rear or back part or view of an object, person, scene, etc.; that part which is opposite the front.
2. rump; buttocks.

noun

1. the back of something
2. (ˈbækˌsaɪd) informal the buttocks

n.

c.1400, from back (adj.) + side (n.). In the specific sense of “rump, buttocks” it is first recorded c.1500.