Don't have a cow

don't have a cow (don't become so excited or angry, don't get so worked up; chill out; don't make a fuss, don't make a scene, don't get all upset (whether through panic, anger or something else)
~ не распаляйся, ~ не кипятись;
~ не истери, не паникуй, не устраивай сцен
~ Не...
~ Не надо писать кипятком
(Др. варианты)

Example 1: Don't have a cow—it's no big deal.
Example 2: Don't have a cow, Mom, I'll clean up my room.

More examples:

My older sister totally had a cow when my friends and I hid under her bed and waited for her to fall asleep so that we could scare the living daylights out of her. When we jumped out, she screamed, covered her head, and started sobbing. When she heard our laughter, she realized what had happened. She gave us a strong cursing and hit us hard with her pillow as she chased us out of her room. One of my friends told her, “Don't have a cow!” and that only angered her more.

...This phrase has always made me angry, too. If I'm upset, then there is a good reason for it. If someone tells me not to have a cow, they are saying that I am making a big deal out of something trivial.
I don't think this phrase should ever be said to someone who is reacting in anger. Sure, you can say it as a comment about how you think someone may react, as long as that person isn't in the room at the time. If you say it to them after they have shown anger, you will only be adding fuel to the flame.

True. The expression is very informal, so it shouldn't be used in situations where serious empathy or counseling would be in order. The advice to not have a cow might best be used in circumstances where the initial situation sounds worse than it actually is. When a child brings home a bad report card, for instance, a parent might react badly at first, but the long-term problem can still be addressed. (wisegeek.org)

Related vocabulary:
[have a cow]
throw a tantrum
go postal
go ballistic
freak out
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Фраза стала популярна после мультсериала «Симпсоны». The cartoon character Bart Simpson helped to popularize this expression with one of his catchphrases, "Don't have a cow, man."