Why is 'x' the unknown?

When The Arab Muslims invented Algebra they called the unknown factor in an equation as "الشیئ" meaning "a thing". When Spanish translated the Algebra to their language they didn't have the word to represent the sound for "ش" hence they borrowed the Greek alternative for it called "kai" written as "ϗ". Later when Algebra was translated to English they took the unknown factor  "ϗ" as "x". And that's how we got the unknown "x".