Omar Khayyam, philosopher, mathematician, astronomer, wine specialist and a poet (see The Rubáiyát of Omar Khayyám) went the distance while he was working on algebra to unveil the numerical mysteries.
He referred to the unknown variable as "thing", because no one had before thought about the variable deeply enough to need to name it. The Arabic word for thing is "shay". As in Turkish it is "şey". Khayyam went on using shay for the unknown variable in the rest of his works.
In 756 A.D., when The Umayyad Caliphs conquered Andalucia, Spain, scientific translations between Arabic and Spanish began, too. While Khayyam's works were being translated, the translators couldn't make out what to do with this word shay. In order not to manipulate Khayyam's original idea, they decided to leave the word as it is. But in Spanish the vocalization of "sh" could be written with the "x" letter. So they came out with the word "xay" to call the variable.
The further works and studies on [mathematics] and algebra in the western world simplified the word xay into a single letter "x".
The unknown variable "x"…