A mismatched and potentially blasphemous term, short for "Jesus Christ, mother of Mary!", which without reflection often passes uncorrected.
A good example of how seldom people listen to themselves or others when they curse.
Could also be Mary's mother. Whomever Mary is, she should be quite worried.
Person A: "Holy crap! That bus is on fire!"
Person B: *Turns to look* "Jesus Christ, Mother of Mary!"
Person A: "...Dude, what the hell?"