Pronounced "b" + "ass". Derived from the compounding of the words "back" and "ass".
A "bass" in this sense is the term to say that you can't tell where someone's back ends and ass begins. This can be caused by having a 1) a flat ass, 2) lower back fat that makes it hard to tell where the back stops and the ass begins. 3) both
Antonym: J-Lo booty.
It is pronounced like the instrument, not the fish. Pronouncing it in the latter sense is a source of ridicule.