Something you say when someone is being annoying. When you respond with "Chill a bitch", you are basically telling the person to chill out and shut the fuck up.
Jane: OMG, I am so scared. We are watching Chainsaw Massacre, omg is this a true story? Omg~
Adam: Chill a bitch. Seriously.
Jane: Is it a true story, tho?
Adam: Heh heh.