Mary, the mother of Jesus. This is typically an exclamation of disbelief, usually targeted at something so out there that it's hard to believe or process. You could also say "Holy shit!" -- it's quicker.
Sweet mother of Jesus! Did you see that chick just crush those beer cans with her 44HHH tits?!?!
A statement similar to Jesus-Fucking-Christ that is often used in a situation of utter disbelief, shock or frustration. It is arguably blasphemy as well. Can be announced slowly and clearly to express disgust and hopelessness, or loud and fast to alert someone of an imminent disaster that will most likely end in catastrophe.
Also might be a sign of Tourette syndrome.
Jesus-Titty-Ass-Mother-Fucking-Christ! I almostdied!