1. A letter sent by a girlfriend (usually) explaining to her guy that she has no wish to see him any more, or that the relationship is not working out. Equivalent when the target is female is the dear
Jeannie letter.
2. Letter from a would-be
employer regretfully informing a job applicant that for one reason or another, which
may or may not be given, they didn't make it.
Marie, not being the most
confident girl on the block, gave
Desmond her dear John letter after he ogled too many women on the beach.
I just got the dear John letter from ToyMaster last week.