Why is X Used to Symbolise the Unknown?

The letter X has been used to symbolise mystery or the unknown in Western culture for the past 600 years. But why?

[redtrychguy]

So sad that YouTube didnt promote that vid

[arielsuryaboiardi]

Very good and intriguing video, ad usual: my best congratulations. Anyway, just a bit of classicist's pedantry: the name for the χ character is "chi" (pronounced /kiː/). Thanks for the research you did, I hope this channel will take off soon!

[iNnjaX]

My man ripped off a ted talk almost word for word. Damn

[killingspeerx]

i remember when we were at collage and our doctor asked us to prove or get a specific number for the equation, so most of us used to choose letter X as an unknown nd he used to tell us "So we can only use letter x? what about other letters? or is it because you are only good at memoriesing that you don't use any other letter except x? how do you want to be an engineers with your skills"

good old days and quite funny video

[tinajabry]

How elegantly ripped off from Terry Moore's ted talk