Being afflicted by or suffering from an extreme case of Derp.
Those found afflicted by Derpitude are known as 'Derpes', because retardation starts with the parents and their act of breeding causing the sexually transmitted affliction which develops into Derpitude in the later stages of life.
If you, or someone you know, are diagnosed with Derpitude, please, for the love of god, don't breed.
Guy 1: Dude, what is his problem? He's beyond stupid..
Guy 2: We was diagnosed with Derpitude..he's hopeless, man.