Holy cow

was this only a modern phrase..

I don't know. Here's what I found: "Holy smoke!, 1889; Holy cats!, Holy mackerel!, both 1803; Holy Moses!, 1906, Holy cow!, 1942." From I Hear America Talking: An Illustrated History of American Words and Phrases by Stuart Berg Flexner (Von Nostrand Reinhold Co., New York, 1976).

It would seem to possibly refer to India's consideration of the cow as holy. A man from India explained to me that the cow was considered holy in old times because the mothers, a lot of times, died during childbirth. The cows took over (with their milk, of course) as a sort of surrogate mother. Of course, I have basis as to whether this is factual, but it appeased my curiosity for the time.