Indicating retardation in both speech and thought; incomprehensible guttoral sounds intended as English predominantly spoken in the South. See also Southern Accent
Southerner: "Derp-de-DERP! Twang, twannnggg......"
Northerner: "WTF did he just say???"
Southerner: "Derp-de-DOOOO!!! Heheheh!"
Northerner: "Fuck it, I'll find I-95 on a map. Thanks anyway."