Don't have a cow is a humorous cautionary phrase, dating from the late 1950s at least, possibly of US origin. The metaphoric warning means "don't get so worked up." If one pauses to reflect on the physical aspects of a human being giving birth to a full-grown cow, one can see what this phrase alludes to in terms of emotional expressiveness. Prior to "Don't have a cow," the phrase was "Don't have kittens" (or "Don't have a cat"). A variation in the past tense is also used when someone has already become worked up, e.g. "She had a cow when she found out he'd been smoking."

Bart Simpson

Bart Simpson

Bartholomew JoJo "Bart" Simpson is a fictional main character in the animated television series The Simpsons and part of the Simpson family. He is voiced by actress Nancy Cartwright and first appeared on television in The Tracey Ullman Show short "Good Night" on April 19, 1987...

 used the phrase "Don't have a cow, man!" as a catchphrase in early episodes of The Simpsons

The Simpsons

The Simpsons is an American animated sitcom created by Matt Groening for the Fox Broadcasting Company. The series is a satirical parody of a middle class American lifestyle epitomized by its family of the same name, which consists of Homer, Marge, Bart, Lisa and Maggie...

. Shaggy Rogers from Scooby Doo uses this phrase often.