Other forms: had a cow; has a cow; having a cow
If you borrow your friend's brand-new laptop and spill milk all over it so that it no longer runs, they might have a cow when you tell them: They'll likely be very upset, ask you all kinds of angry questions, and maybe even yell.
Have a cow is a bizarre idiom! It seems to have originated from the slang use of cow to mean a fit or tantrum, a usage found in the early 1900s. Why a cow? Perhaps because cattle are easily startled, and when they are, they can sometimes act pretty wild. In the 1930s, the phrase have a cow emerged, and by about a decade or two later, it was quite common. It's still slangy, though: Don't tell your teacher not to have a cow.