Once upon a time, Mr. X and Mrs. X went on a holiday to Amsterdam, the town whose crest bears three Xs. They took their XXL suitcase. They wanted to have an X-tra special time, so toured the city in stead of going to the cinema, although X-Men and X-Terminator were on.
Whether to signify something secretive or unknown, or rather something interchangeable, 'X' can always be used as a variable or shortener. It came to be used in writing in three different forms: as a letter, a symbol and as a digit.
The letter X's ancestor is the greek letter X (Chi) and is one of the easiest and fastest letters to write. Barmen used to write it on the back of their hands to signify that minors were excluded from alcohol consumption. Sure enough, this letter thus became the symbol of Straight Edge Punk. And analphabets continue to sign their documents XXX.
'X' is also widely used as a symbol, signifying for example the 'Christ' in x-mas, but also all sorts of crosses and crossings, be they of paths, in genetics or horse-breeding.
The Roman digit X for '10' was put together out of two Vs (or Us, depending on the handwriting), the Roman '5', whereby one stood on its head. It was enough to have messy handwriting, however, and you'd be guilty of ein X für ein U vormachen, a German saying literally meaning 'using an X instead of a U', and figuratively, having somebody on.
In any case, in speech the 'X' always takes a vowel, which in English allows for a spice up of words like 'extreme' which becomes x-treme and can even lift the gloom off things like X-boyfriends.
And what ever became of Mr. And Mrs. X? They sent a postcard with XXX from XXX ('three kisses from Amsterdam') and left the room maid a tip with a Thx! note.