It basically means to chill out, and not make a big deal about something.
Like "don't have a cow" only cooler.
Ex:
Amanda: "I am sick of always being the one to clean up around here! You'd better start cleaning up after yourself or I'm going to sleep with your brother again."

Jason: "Fineee damn, don't bust a herpe over it."
Jason: "And leave my brother out of this!"
by MyRealNameIsJason December 11, 2009
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