This is the noise of a sad trombone, usually lowering in pitch after each 'wah,' that sounds off when someone whines or something bad happens. The last 'wah' sometimes is spelled with extra Hs or capitalizations, in order to drag out the last and saddest sounding 'wah.'
Interchangable with a fail, a boo hoo, or finger tear drops.
Jason: d00d that girl just shot me down.
Aimee: wah wah wahhh!
Jer: I wanted to go to the show tonight but then I got grounded because my mom caught me watching pr0n.
Craig: wah wah WAH!
Onimonapia (meaning word that is spelled like a sound) that is supposed to stand for 70's guitar Porn-riff. Usually this is the music that came on when the action was just about to get started. It has been used in a subcultural context to denote any porn-like or pornified situation.
Your friend picks up an extra large sausage. This is agreat time to sing Bow Chicka Wah Wah.
You see a hot guy/girl that you could imagine yourself in a porn with. Also a good time to sing Bow Chicka Wah Wah.