Why Is “X” The Unknown?
“File:Latin capital letter X with descender.svg” by Person or Persons Unknown is licensed under CC BY-SA 4.0
The term “x” has always been the term for the unknown or hidden. This letter in our alphabet in used often in our lives such as Project X; the X-Men; the X-Files; and most commonly the variable we use in math equations. Everyone who has had the very fortunate experience of taking algebra has come across variable’s before. Occasionally when we need more than a single variable we will use different letters, but have you ever wondered why we always come to “x” first?
Why x? why not a different letter in the alphabet? So why is it that the letter “x” represents the unknown?”
The Arabian language has existed for over 1000 years and from the Arabs, we get a system they use called al’jebr. Al’jebr roughly translates to “the system for reconciling disparate parts. When this system was introduced to the English language, we took it and now understand it as “algebra”
When the Arabic texts made its way to Spain in the eleventh and twelfth centuries, there was immense curiosity at interpreting the language into one the Europeans can understand. Many problems were met along the way but one of the main ones had to do with translating the mathematical terms the Arabs used to ones that fit in the Spanish pronunciation box. These sounds also usually arent represented by the characters that were accessible in the European language.
For example: this is the letter sheen: 
it makes a sh sound as if you were shushing someone.
It also happens to be the first letter of the word shayun: شىء. This word means something. Exactly as we use it in English meaning some unknown thing.
In the Arabic language when we want to make the word definite we add the word “al” creating al-shayun which means “the unknown thing”.
This word is used in early Arabic mathematics and needed to be translated into the Spanish language. The only problem was you couldn’t pronounce the sound “sh” in the Spanish language. Instead the medieval scholars tasked with this job took the “ck” sound from the classical Greek language in the form of the letter kai. 
Some time later when algebra was translated to Latin, the Greek “Kai” was simply replaced with the Latin “x”. One Algebra had reached Latin translation, it formed the basic language in our mathematical textbooks we use nowadays.
The answer to why “x” is always known as the unknown is because you can’t pronounce “sh” in Spanish.
