Originally Posted by
kanderson1965
It is all down to the the early days of the telegraph where messages were paid by the letter. To avoid unnecessary costs words were often abbreviated, X was chosen to end the abbreviation as it was an uncommon letter in normal use. This probably started informally amongst the telegraph operators who were incentivised as they were paid per message sent.
I think this is as close as we're likely to come. The need to minimize keystrokes led to abbreviations, and X is an uncommon letter unlikely to be confused with anything other than an abbreviation.