This is a short, fun and informative TED talk by Terry Moore about the origin of the symbol ‘x’ as the representation of the unknown (or variable) in algebra. Reminding us, thus, that it is through its use in mathematics that we took the ‘x’ to commonly mean ‘an unspecified or unknown thing or person’ in our (use of) life.
I thought, this mention on the Oxford Dictionary was also interesting:
“the introduction of x, y, and z as symbols of unknown quantities is due to Descartes (Géométrie, 1637), who took z as the first unknown and then proceeded backwards in the alphabet”
Oxford Online Dictionary definition, under its dictionary entry for ‘x’ as a noun.