x is the european transcription of the arabic word شيء (pronounce 'chi', or 'chaï'. or 'tchi', depending on the accent). This word means 'thing', or 'something' and was used by first arab and iranian mathematicians as a symbol to designate an unknown or unfixed value in a mathematical expression.
Back to these times, 'x' might have been considered by translators and european mathematicians as the closest approximation to this symbol of the unknown.
