When a pretty girl hits you in the back of the head and threatens to kick you in your "THORAX" because you obviously did something to piss her off even though you're really sorry. I am assuming thorax refers to one's sternum or to be more precise... the part of the human body between the neck and the diaphragm and/or the corresponding part in other vertebrates. Let me put it this way, your THORAX is a body part that will experience a great deal of pain if you piss this girl off.
(Talking to me) "I'll kick you in your THORAX". --That Pretty Girl