There were many times in high school when I felt that all of algebra would remain an unknown to me. That formidable looking X seemed to represent a crossroads where I was always got lost. But now I think I've turned the corner.

It was Renee Descartes, the 17th century philosopher and mathematician, who began the use of letters at the end of the alphabet to represent unknown quantities, with X as the choice for the first unknown. Why X and not Y or Z? He never gave an official reason (he was too busy planning to torture high-schoolers... I mean, enlightening the world), most scholars agree that the widespread adoption of the X is, in spirit, just the opposite of what algebra always represented for me: ease and simplicity. One could write the letter X quickly and it was more likely to be legible than other letters.

Source: HOW DO ASTRONAUTS SCRATCH AN ITCH? by David Feldman