from Wiktionary, Creative Commons Attribution/Share-Alike License
- abbr. An abbreviation for hugs and kisses, usually placed at the end of a letter.
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. (Wiktionary)