Why is ‘X’ the unknown?

The letter “X” is used to represent unknown thing, and many people use it without knowing the secret behind it. From his study, Terry Moore has discovered that the secret of that goes back centuries, when Arab invaded Spain and brought with them huge amount of information. That information was combined of many different subjects, such as Math, Algebra, and medicine. In Algebra Arabs used to say “Shi-on” to represent undefined thing. This Arabic word was simplified by the letter “X”, because Spanish Language does not have any letter that sound (sh). So if there is a sound “sh” letter in Spanish, the undefined thing may be represented in a different way.  I think that those who study linguistic may know that there is some scientific vocabulary that is used nowadays was derived from Arabic Language via Spanish. For instance, chemistry, alcohol, algorithm, arsenal, lemon, magazine, and massage, all these words and many others were borrowed from Arabic Language.