holy cow
Also, holy mackerel or Moses or moly or smoke. An exclamation of surprise, astonishment, delight, or dismay, as in Holy cow, I forgot the wine, or Holy mackerel, you won! or Holy Moses, here comes the teacher! or Holy smoke, I didn't know you were here too. The oldest of these slangy expletives uses mackerel, dating from about 1800; the one with Moses dates from about 1850 and cow from about 1920. None has any literal significance, and moly is a neologism devised to rhyme with "holy" and possibly a euphemism for "Moses."
Source: The American Heritage� Dictionary of Idioms by Christine Ammer.
Copyright � 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company.