Herb Schaltegger wrote:
I had a friend in college who was a geology major. He says there is a
place in our state (Tennessee) that is named Bumpas because when it was
first settled, the hilly, shale-covered terrain caused lots of slips,
slides, falls and broken bones. Being the hard-working yet-uneducated
folks that they were, they named their little village . . . yep, you
guessed it . . . "Bump Ass". As the town expanded and got
semi-civilized, the newcomers were shocked and eventually the town's
name was gentrified into Bumpas.

Or so my friend swears . . .

It comes from the French bon pas, which means good step.
Bumpas/Bumpus are also not too uncommon British and
French surnames.