When a person is overreacting in a situation where it is completely unwarranted, one can be told to calm their shit. This can also be used to describe redundancy. Basically, someone is being annoying, a spaz, weird, or just plain being a dbag, you may use this phrase.
A kid crosses the street when the no walk sign was flashing and a car almost hits him.
Dumbass kid: WTF? That fucker almost hit me!
Bystander: Calm your shit! You shouldn't have been in the road!
Dumbass kid: WTF? That fucker almost hit me!
Bystander: Calm your shit! You shouldn't have been in the road!
by AlexandriaC February 11, 2009