An onomatopoetic phrase stated out loud when one has a
boner.
Typically, the second 'bump' is stated with higher inflection, and the volume of the phrase is representative on how raging the
boner is.
The phrase can also be used in general situations of extreme
happiness.
After seeing a tall blonde girl with huge tits walk past you while sitting on the park bench with a couple of buddies: "bump bump"
"Hey bro, I just saw that hot ging at that
party, and I totally bump bumped."
"Tiger Woods just won the tournament, our
douchebag varsity golf roommate
Patrick probably just bump bumped."