How X Came To Mean Kiss
Since 1883 when Robert Louis Stevenson wrote about a treasure map where X marked the spot of buried riches, that alphabetic letter symbolized more than a part of language. The symbol segued into other, not always benevolent, meanings. For example, back when photojournalism began, an X indicated the place where a crime occurred (per Idiom Origins), particularly with regard to corpses. The practice inspired the slang phrase, "spotted," to indicate the word "murdered" (via Word Origin Stories). The British Army would also mark a piece of paper with a black x and pin it over the heart of someone to show which prisoners were sentenced to death, said The Idioms.
For Christianity, the X became an important part of the religion's iconography. "X meant Christ, and because of that, it meant faith and fidelity," says Marcel Danesi, a professor of linguistic anthropology and semiotics at the University of Toronto said to the Washington Post. "We still see it on churches from medieval times." Before people could write, X also served as their signature, and then later the symbol evolved to represent a kiss.
How X met O
During the Middle Ages, an X as a signature often marked letters as well as oaths to show loyalty and love, with the sender kissing the mark to assure their word. The habit became a precursor to the acronym SWAK ("sealed with a kiss"), something World War I soldiers inscribed on their envelopes home. "Symbols have a way of jumping from one domain to another," said Danesi to the Washington Post.
When X found O is still open for speculation. Some point to tic-tac-toe as the origin of the coupling. The game started during ancient Egyptian and Roman times and first used pebbles or coins. "These are two of the simplest contrasting symbols, easy to master by illiterate people," said David Parlett to the media organization, author of "The Oxford History of Board Games." The earliest Stephen Goranson, a Duke University researcher, could find the two together was a letter to Santa printed in the Fort Pierce News Tribune from November 22, 1960, which ended with "Love & Kisses XOXOXO DAVY MIKEY & CHERYL."
Whatever the origin, XOXO now indicates an expression of love, said Brides, and can be used casually in an email to a good friend as a way to say goodbye or to indicate a more undying type of romance in an anniversary card. Or as a sign-off in an article that wishes you a Happy Valentine's Day. XOXO!