A universal nickname for that really annoying girl that everyone hates. She could be your clingy ex-girlfriend or the girl in your government class that always asks stupid questions. If she's "Derpy" then chances are that everyone you know can find something they hate about her.
"Dude, derpy showed up at my house yesterday, she's so creepy!"
"Are you talking about that blonde chick with the annoying voice? I hate that chick, she gets on my nerves."