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.
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!