Someone who has shit their brains out.
-Is Justin still on the bowl?
-Yeah, and he's unresponsive.
-Maybe he's finally gone full diarretard.
A person or individual who's intelligence ranks them beneath one who is retarded.
Person 1: "Man, Jenny is so hot, but she is so incredibly stupid. I wonder how she remembers to breathe. She is a diarretard"
Person 2: "Yes she is. Please pass me the meth."