Its a contradiction with the same meaning as holy crap. It is an interjection that is used when something isn't right or you forgot something to express your angst.
1. Beats the hell out of any other way of expressing surprise, happiness, or generally anything
2. Has no logical meaning so don't ask
3. I JUST SAID DON'T FUCKING ASK
4. It's what Jesus says to Colonel Sanders in heaven. Bitch.