We use "x" to represent unknown quantities all the time. It appears in algebra (solve for x), but also in culture (The X-Files, X-Men, X Prize, and so on). But how did we collectively settle on that one mysterious letter?
In this four-minute video, Terry Moore takes us on a brief linguistic journey explaining how this came to be. Have a look:
If you want the shortest possible version, the progression is from the Arabic شيء for "the unknown thing" to the Greek χ and then to the Latin x. If you'd like to see the transcript or different subtitles, head over to the TED site.
Note: Moore gave a talk some years back on how to tie your shoes. Worth a look!