When a person can't get himself to talk about a certain ongoing subject (normally on chatrooms) and is put in a awkward situation, where some might expect a reaction from him and he goes,
It comes from "so done with this". Usually related with Otaku culture for some reason.
Person 1: I just raped this bitch so fucking good, i swear i did.
Person 2: Hahaha so funny!
Person 3. o.o.. dun dun