Buttocks and Back end

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

Back end and buttocks are semantically related in rear topic. In some cases you can use "Back end" instead a noun "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. You can use "Buttocks" instead a noun phrase "Back end".