exclamation. A sweeter, more motherly, more seafaring version of "Holy shit!"
Related terms: holy shit on a stick, sweet merciful Buddha on a crutch, holy freakin' Christ in a Christmas tree, Jesus H. Christ on a popsicle stick.
Real citation: "Except for the being in love with Lauren part, because sweet Mother Mary on a motorboat, I had an aneurysm at that point. Literally missed the next couple of lines. Sad thing is I'm going to have to go back and rewatch to see if I think he was faking Vaughn out or not. And what's with Vaughn and Sark being each other's great enemies? Seriously, that's just lame and I don't buy it."
(JenMD, Feb. 23, 2005, Television Without Pity, http://forums.televisionwithoutpity.com/index.php?showtopic=3123657&view=findpost&p=2562476)
Made-up citation: "Sweet merciful Mother Mary Magdalene on a motherhumpin' motorboat--I am gonna get some cheap Peking duck this weekend. Woo and fucking hoo! And quaaaack!"