There are a few different theories about its origin:
* The most common theory is that it came from the reaction of a cow when it is agitated or upset, as they tend to moo loudly and thrash around. This is a metaphorical expression where a person's over-the-top reaction is compared to a cow's behavior.
* Another theory suggests that the phrase originated from cowboy slang, where "cow" referred to a person and "having a cow" meant being angry or upset.
* A third theory claims that the phrase was used by soldiers during World War II to describe the sound of bombing raids, which sounded like a cow mooing.
While the exact origin of "have a cow" remains unclear, it's safe to say that the phrase has been used for decades to describe someone who is extremely upset, panicked, or hysterical.