X X X
Meaning: A kiss, at the end of a letter.
Origin: In medieval times, when most people were illiterate, “contracts were not considered legal until each signer included St. Andrew’s cross after their name.” (Or instead of a signature, if the signer couldn’t write.) To prove their sincerity, signers were then required to kiss the X. “Throughout the centuries this custom faded out, but the letter X [became associated] with a kiss.” This is also probably where the phrase “sealed with a kiss” comes from. (From I’ve Got Goose Pimples, by Martin Vanoni)

Neatorama has a great collection of common phrase origins like the one above.