Back end and Buttocks

Back end noun - The side of an object that is opposite its front.

Buttocks and back end are semantically related in back or end part topic. In some cases you can use "Buttocks" instead a noun phrase "Back end".

Buttocks noun - The part of the body upon which someone sits.

Back end and buttocks are semantically related in rear topic. You can use "Back end" instead a noun "Buttocks".