Ha ha, i think that it is just a stupid made up word that people use to sound funny... My sister and my cousin use "derp" and "herp" all of the time just to annoy people so i don't think it means anything
It is just what to say when you have nothing to say. Derp also refers to someone in a rage comic usually the creator, who does not want to disclose their actual name. Also when there is a picture that is so random and funny words can not describe it, derp also works there. It's a hard word to pin down any sort of definition for.