How ‘X’ Came to Mean ‘Kiss’. The idea of using a kiss to sign off on valentines also has a long history, according to the Washington Post.
The use of “X” came to represent Christianity, or the cross, in the Middle Ages. During the same time, the symbol was used to sign off on documents. After marking with an X, the writer would often kiss the mark as a sign of their oath. As the gesture grew among kings and commoners to certify books, letters and paperwork, these records were described as having been “sealed with a kiss.”