Well class...
etymologically "bottom" derives from Old English "botm, bodan," meaning "soil, ground, lowest part".
First used in modern sense only in 1794.
We SIT on it, on the ground. It's only our middle when we're standing...There could be a muckier, scatological explanation but that wouldn't be appropriate here.

jefesse:
I bet alliteration has something to do with it: Bum. Butt. Behind. Backside. Buttock. Bottom. Bare.
jefesse's point about alliteration is noteworthy, and which letter do all these words begin with? And what does a capital B look like on its side...?
Now, for homework...