An expression used when a person, or yourself, has done somthing extremely stupid and dopey Person 1: Dude, I just hit my head on the door. Person 2: HERP DERP. Another way of saying duh. Stacey: I ordered the food, it will be here in 10 minutes. Louise: okay when will it be here? Stacey:herpderpderpderpdurrrr.