Used when telling someone to calm the fuck down now. If the situation is not en emergency the N may be dropped. In a situation that is out of control you can also add OE (or else), OD (or die), or OMO (or move on) to the end to make it quite apparent that you will calm down or get out my face.
Calvin: Get off the damn counter before i skin you.
Hobbs: Woah! CTFDN!
Calvin: What did you just say to me pussy?
Hobbs: Let me repeat that CTFDNOE.
Calvin: OE what?