Not "all there in the head"; possibly lacking intelligence or common sense. The wall-eyed background character in "My Little Pony: Friendship is Magic", Derpy Hooves, is named after this.
Hey, look at that guy sticking his aluminum foil-wrapped dick into an electrical outlet. That's derpy if I've ever seen it.
Derpy is best pony.
In the context of Speedrunning, this is often said in a joking or sarcastic manner when a trick or other act takes more than one attempt.
Speedrunner: *Fails to make a jump four times, then finally succeeds in making the jump*
Speedrunner: Hey, first try!
