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1.
The REAL definition behind X's and O's is this
The first mention in literature of XXX for kisses at the bottom of a letter was in 1901, according to the Oxford English Dictionary. The X itself is very old.

The custom goes back to the early Christian era, when a cross mark or "X" was the same as a sworn oath. The cross referred to the cross of Calvary and the first letter of the Greek word for Christ, Xristos.

Even as little as a hundred and fifty years ago, not many people could read or write. The "X" at the bottom of a document took the place of a signature. They would kiss the "X" as a crucifix or bible was kissed to emphasize the importance of the mark. It was this practice that lead to the "X" representing a kiss.

O is an American thing and represents the arms wrapping around someone.
the person above me believes that everytime a girl signed a letter O that he was getting a kiss, which is why he'll never get laid. X's and O's
by Abennobashii May 13, 2008
 
2.
1. symbols used to represent sports players on a chart

2. hugs and kisses
The Xs and Os looked a bit disorganized to me.
by The Return Of Light Joker October 27, 2007
 
3.
Hugs and kisses. 'X' is for the crossing of the arms when one hugs another, and 'O' is for the shape of one's mouth for a kiss.
Dear Love-of-my-life,
(insert really sappy love letter-type thing here)
X's and O's,
(insert the sappy love letter's writer's name here)
by Jessica Myers June 20, 2006