Noun. Someone who is slow, spasticated, handicapped or in any way mentally retarded.
Also an exclamation at one's own stupidity or the stupidity of a close party. Normally "hurpa-durpa-durp" (with added '-adurp's for added emphasis). Must be said in the fashion of a mongoloid.
Not to be confused with "hinga-dinga-durgen".
A: "Is like a turtle and a tortoise different cuz ones a reptile and da ova is a bird?"
B: "Stop being such a hurpadurp."
A: "OMG! Them things are liek only £20!"
B: "No, it says they're £200!"
A: "O right, I see... hurpa-durpa-durpa-durpa!!! *X-z*"