Very similar to the meaning of derp. Has a hidden meaning of being awesome. If you are a derp, you suck and you have no hope. If you are a darp, you still suck but you have hope. It can literally mean anything though. Darp is what you make it.
Man, everyone has been catching the darp fever these days.
An exclamation following an instance of high-class stupidity. Like saying Herp Derp, only classier and more pretentious. Hand movements pantomiming a harp player are optional.
Melba: Oh hey, how's it going?
Numps: Can't complain. Oh, jeezum crow! I just stepped on a tack!
Melba: Haha! HARP DARP!
Numps: You classy bitch! Stop plucking those invisible strings!