Uhm, from what I remember, something involving South Park brought it to light. A guy kept hitting his head and saying 'derp' while doing it. I think.
But Derp is like a, well it has several meanings from what I gather, and it's like...
Someone can be a derp - meaning they're weird, out of the ball park kinda way, funny and just plain random.
It also describes an action I suppose. Like a 'post-retardedness comment' when someone done something stupid and easy to figure out. As in asking a question that's easy to answer.
It can also describe a face
And that's all I remember. But from what SP did, I'm pretty sure it's the 2nd definition. So yeah. Hope that sheds some light.