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.