“Dear John letter” refers to a letter written by a woman to her husband or boyfriend to inform him that their relationship is over, usually due to the woman finding another man.
The roles may also be reversed, with a man notifying his wife or girlfriend with a “Dear Jane letter”.
Story behind the Letters
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” would instantly be aware of the letter’s purpose.
So Ladies & Gentlemen, behave yourself well to your partner to avoid Dear John/Jane letters. 😉