Alternative forms 
From use of X/x to mean “kiss” and O/o to mean “hug”. Use of “X” to indicate a kiss attested since 1763, preceded by medieval use of “X”, which was then kissed, by illiterates to indicate a signature. Use of “O” is more recent, and presumably created by analogy (e.g., X/O in tic-tac-toe). Speculative theories on precise origins abound.
See also 
- ^ OED: “X” 1763 Gilbert White Letter (1901) I. vii. 132, I am with many a xxxxxxx and many a Pater noster and Ave Maria, Gil. White.
- ^ “X” in Douglas Harper, Online Etymology Dictionary (2001).
- ^ How Stuff Works: How Valentine's Day Works
xoxo (using Raguileo Alphabet)
- Wixaleyiñ: Mapucezugun-wigkazugun pici hemvlcijka (Wixaleyiñ: Small Mapudungun-Spanish dictionary), Beretta, Marta; Cañumil, Dario; Cañumil, Tulio, 2008.