An expression of sheer and utter disbelief. Derived from the more common expression "Holy Cow!" (which is of Hindi origins).
Etymologists have tracked the phrase to a specific household in Colorado, where two (and perhaps more) males have been recorded using the expression. It is rumoured that the expression will soon inexplicably spread throughout much of Cambridge, MA.
"Holy Cats! I still have a lot of work left to do on my thesis!" quipped one of said Colorado natives.