What you say when "God damn it" just doesn't suffice-- you need to damn it even harder. First used by Vegeta in Team Four Star's Dragon Ball Z Abridged, it has come to be commonly used to express one's inner torment.
Ross: Hey you guys makin' a commercial?
Danny: That is a sweet shirt Ross, but it isn't gonna make our dinner any faster, that is a sweet shirt.
Ross: Oh. Okay.
Arin: *whispers* God damn it Ross...