dont have a cow

Dont have a cow is an idiom which is advising someone not to get so worked up as in upset or agitated over something that shouldnt be such a big deal. The phrase have a cow comes from the fact that a human being giving birth to a full-grown cow in all of its size and weight would cause a large amount of emotional expressiveness; in this way, the person is advised not to overreact to something superficial or meaningless.