a derp is an unflattering photo of a person's contorted face taken by mistake
usually makes the person seeing the photo laugh and the person in the photo cry with shame
Something you say when:
-Someone does/says somthing stupid
-You'e stuck with a noob (n00b)
-You are stuck doing something absolutely fucking horrible
-You come into contact with derp or anything derpy.
-derpo: What does ice cream taste like?
you: Derrrrrp