The Meaning of Life, the Universe, and Everything.
Yeah, "DERP" originated in the movie Baseketball used in place of the word "opps." Since then the internet has twisted the word's meaning to someone stoopid and crossed-eyed imagery has been associated with it.
According to bronies, derp means clumsy, facial problems. Derpy, a character from My Little Pony, is a character who is very clumsy and can't quite do a lot of physical things, like holding things and being careful. She also has eye problems and one eye goes up and one eye goes down, in short, she's permanently crosseyed and pretty much derp means clumsy and visually disabled.