dear John letter at English => English (Longman) Of Explained:
n [C] old-fashioned a letter that a man receives from his wife or girlfriend that tells him she no longer loves him//
Dear John letter at English (WD) Of Explained:
EtymologyWhile the exact origins of the phrase are unknown, it is commonly believed to have been invented by Americans during World War II. Large numbers of American troops were stationed overseas for many months or years, and as time passed many of their wives or girlfriends decided to begin a relationship with a new man rather than wait for their old one to return. As letters to servicemen from wives or girlfriends back home would typically contain affectionate language, a serviceman receiving a note beginning with a curt "Dear John" (as opposed to the expected "Dear Johnny", "My dearest John" or simply "Darling" for example) would instantly be aware of the letter's purpose.
There are a number of theories on why the name John is used rather than any other. For starters, John was a common name in America at the time. John is also the name used in many other terms that refer to an anonymous man or men, such as Inter: term » John Doe or Inter: term » John Q. Public. Further, there existed prior to World War II a radio programme starring Irene Rich which was presented as a letter written by a gossipy female character to her never-identified romantic interest. It was both titled and opened with the words "Dear John", and may have contributed to the genesis of the term.