It can be used as a signature that is printed in lieu of an individual’s signature…”Typically, individuals sign their full names when executing legal documents. Sometimes, however, individuals use only their initials or other identifying mark. For illiterate, incompetent, or disabled people, this mark is often the letter X. Documents signed with an X sometimes raise questions as to their validity and enforceability.”
Related, the X refers to a kiss. “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.” (Which is why Xmas does not bother me in the slighest; it actually has sacred roots.)
“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.”
Yet, X-rated is “vulgar, obscene, or explicit in the treatment of sex: an X-rated novel; X-rated graffiti.”
X is an algebraic meaning ‘unknown quantity’ (1660 in English). Yet X also means the specific, known location of the buried treasure; “X marks the spot.”