Origin of the x symbol in algebra

In algebra, x is symbol for a number with unknown value, commonly denotes an unknown variable. Although other symbols are applicable, x remain the most common symbol. How is the origin of this symbol of x.

Muhammad ibn Musa Al-Khwarizmi, in his book Algebra, used the word شَيْءٌ (spelled sha-i-oon), meaning thing or something, for an unknown variable. This was taken to Old Spanish with pronunciation “šei,” which was written xei, and soon abbreviated to x. Spanish pronunciation of x changed since this, x is pronounced like ش in Arabic (the influence reached English too, x in first letter of a word is pronounced like ش, like in xylem, Xerox, xenon) . Other sources say that this x is an abbreviation of Latin causa, translated form Arabic شيء. This habit was the origin of using letters to represents quantities in algebra.