here's another phrase i'd like to understand more~~ ^^

It mean's don't overreact.

I never heard this expression in my life. I don't know what it means. What I do know it that whoever says this does not express themselves well. Why not go back and ask the person who said it what they mean?

Don't have a cow is a humorous cautionary phrase,dating from the late 1950s at least ,possibly os US origin.The metaphoric warning means "don't get so worked up".It one pause 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 whensomeone has already become worked up, e.g. "She had a cow when she found out he'd been smoking."

It's the same as " don't make a mountain out of a molehill" don't make it a big deal.. don't over exagerate..

"Don't have a cow!" Means to not get over exited or to not go over board on something. For example Lets say one of your friends tells you they are going out with this person that you like but have never talked to. If you get mad at them and start a fight they may say "Don't have a cow!"