You might receive one of these when you are getting dumped by your erstwhile partner. I don't think it is a coincidence that the a recent movie called Dear John includes a Dear John letter as one of its plot points.
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According to Wikipedia:
Here's another source that concurs:
Early in Anthony Trollope's 1864 novel "Can you Forgive Her?" the heroine, Alice Vavasor, writes a letter that is central to the novel's plot development. She breaks off her (second) engagement by writing a letter, addressing her fiancee "Dear John" and then repeating this same greeting several times throughout her letter. So I imagine this very popular London novel was the origin of the concept, and that the idiom was further popularised in the USA during the second World War as suggested.
Here is more information about "Dear John" from Wikipedia:
"A Dear John Letter", or "Dear John" is the name of a popular country music song. It was popularized by Ferlin Husky and Jean Shepard, and was a crossover country-pop hit in 1953.
The song played on the concept of a Dear John letter while referencing the United States' involvement in the Korean War. Here, a young woman, whose boyfriend John was stationed in Korea, writes stating that she is breaking off the relationship. To add to the heartbreak: She is marrying John's brother, Don, and wants her photograph back (because — according to the lyrics — Don wants it now).
If you want to know more about the song lyrics here is the link:http://www.lyricsmode.com/lyrics/f/ferlin_husky/a_dear_john_letter_lyrics.html