At least since
Edgar Allan Poes story
X-ing a Paragrab, 'to X' has been a
verb in English, meaning 'to replace letters in text with the letter X'.
In early Christian times, the X symbol was often used as a Christian emblem - actually standing for the Greek chi, the first letter of the word Christ.
'X' is also the name of the software system that provides GUI capabilities to many Unix and Unix-like systems, such as Linux; its full name is X Window System.