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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."
American Heritage Idioms
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Phrase(s): Holy cow!
Inf. Wow! • Holy cow! I never expected such a nice gift! • Give me a chance! Holy cow, don’t rush me!
McGrawhill's American Idioms And Phrasal Verbs