When I googled it, it mentions a character named "Mr.Derp" who replaced Chef for an episode, and they made up the word to describe "a ditzy, clumsy situation you can see from a mile away, and can't help but laugh". I also remember it having something to do with them making fun of Rob Schneider with a "movie preview" in which he said, "herp de derp" throughout most of the dialogue, but the tone of the dialogue was basically that of a romantic comedy voice over. Someone later used soundbytes to recreate the Kirby theme.

(Whoa, I'm a carrot.)