Variations of the word include derpy, derped, derpette, derpity der hurrrr. Herp is also related to derp and usually follows derp in a derpy interjection. Herping is almost always a verb, and when in combination with derp it does not have to be negative.
Verb: "Wow, that guy just derped everything up"
Adj: "Wow, that guy is derpy..."
Person A: "Derp de durrrr... herp derp?"
Person B: "Herpity derp hurrr... Derp derp?"
Person A: "Hurrr :/ "
-friends laugh and giggle-