What you say the moment you realize you blew it, sometimes even when you didn't screw up beyond. It's always said or thought before an imminent mishap, like falling down the stairs, or breaking something in which case more severe profanity is utilized immediately afterwards. By now you know what I mean. It's also an expression of embarrassment.
Jess started accelerating before we got to the bump so I grabbed the oh crap strap and kissed my ass good-bye. We flew through the fucking air at 70 mph!