“Holy cow!” (and similar) is an exclamation of surprise used mostly in American and Canadian English.
From the Dictionary of American Slang (1960):
“ “Holy Buckets!” Equiv. to “Holy cats!” or “Holy Mike!” both being euphemisms for “Holy Christ!”. This term is considered to be very popular among teenagers, and most teens claim it is definitely a very popular phrase. It is also the common oath and popular exclamation put into the mouths of teenagers by many screenwriters, and, is universally heard on radio, television, and in the movies. It was first popularized by the “Corliss Archer” series of short stories, television programs, and movies, which attempted to show the humorous, homey side of teenage life. ”
Paul Beale (1985), however, in revising Eric Partridge’s A Dictionary of Catch Phrases: British and American, from the Sixteenth Century to the Present Day cites a different origin:
“ The original ‘Captain Marvel’ and ‘Batman’ oaths, ‘holy (something harmless),’ were in turn spoofed in the later 20th century by whatever seemed relevant to the situation. Nigel Rees, in Very Interesting… But Stupid: Catchphrases from the World of Entertainment, 1980, instances “holy flypaper!”, “holy cow!”, “holy felony!”, “holy geography!”, “holy schizophrenia!”, “holy haberdashery!”, etc., and adds, “The prefix ‘holy’ to any exclamation was particularly the province of Batman and [his boy assistant] Robin, characters created by Bob Kane and featured in best-selling comic books for over thirty years before they were portrayed by Adam West and Burt Ward in the TV film series. ”
“Holy cow” was the catchphrase of Yankees shortstop and broadcaster Phil Rizzuto. The phrase was so popular with “Scooter,” as Rizzuto is often called, that when the Yankees honored him decades after he retired, they actually had a real cow with a halo prop on its head. Amusingly enough to all including Rizzuto, Scooter wound up tripping over the cow. Harry Caray, who was the broadcaster for the Cardinals (1945-1969), Athletics (1970), White Sox (1971-1981), and Cubs (1982-1997), also used the phrase, as did 1950s Braves broadcaster Earl Gillespie.
The phrase may have originated with (and certainly was introduced to the baseball lexicon by) reporter and broadcaster Halsey Hall, who worked in Minneapolis from 1919 until his death in 1977. According to Paul Dickson, New Orleans radio announcer Jack Holiday also used the phrase on broadcasts of the minor-league New Orleans Pelicans in the 1930s.
The cartoon strip Common Grounds was originally titled Holey Crullers, a play on this catchphrase.