A 'Dear John letter' is a letter written to end a romantic relationship. The term 'JILT' is a four-letter word meaning to reject or abandon someone suddenly and callously. The five-letter word 'Endit' means to bring something to an end. Finally, the fourteen-letter phrase 'Breakonesheart' is a way of saying that the sender of the Dear John letter is causing emotional pain to the recipient. In summary, the term 'Send a Dear John Letter' refers to the act of ending a romantic relationship by writing a letter that causes emotional pain to the recipient.