A simple, undefined reply when an ignorant comment or action is made. Brought to life in the South Park series, when Mr. Derp made a guest apperance at South Park Elementary as the chef for a day, followed by hitting himself in the head with a hammer and exclaiming "Derp!"
wouldnt it be nice if mods were automatically updated by a code in minecraft that pointed to the changes required for the mod to be updated? what if we could write a code that would do that?? would you want it to be part of the minecraft main release??
Derp is to do something wrong that the normal person wouldn't regularly do. Tipping over a lamp by walking with its chord tied around your leg is a "derp" maneuver. Dropping stuff, not watching where you're going, everything they do on American Chopper. If it's done wrong, and any logical person (or by extension person in a trade) would see and go, "That's wrong" can be considered a derp.
This thread is derpy. Since you could have easily Googled before hand or even hit up Urban Dictionary.