A 'Dear John' letter is a letter sent to a romantic partner to end the relationship. JILT is an acronym for 'justify, inform, leave, and thank, ' which are the four steps typically taken when writing a 'Dear John' letter. Therefore, the phrase 'Send a 'Dear John' letter to' means to write a letter to a romantic partner to end the relationship, following the four steps of justify, inform, leave, and thank.