Originally "wakka wakka wakka" as uttered by Fozzie the Bear of Muppet fame. Fozzie wanted to be a comedian. Unfortunately, he was terrible. The threefold utterance was part of Fozzie's act. The shortened version has gained usage as an expression of delight, exultation or lust (sometimes all at the same time).

When you eat dirt for the first time and leave your family to build a shack in the woods

The sound that Pac-Man makes as he moves across the screen, of course!

the appropriate thing to say if you are a stand-up comedian who happens to be a bear that wears a bow tie and a hat.

The sound of Fozzie Bear's (a character from children's program The Muppet Show) chuckle.

What intoxicated Yalies sing in The Wiffenpoof Song.

Used to describe the sound of somebody being rammed from behind. Often used when somebody makes a reference to somebody "being on the jock" of a rapper.