Why unknowns are usually denoted by "X" ?
More precisely: is this answer really a serious answer or might be a 1 April joke ? Let me sketch it. But please watch it, it is really fun and cool and < 6 min. (Are there any alternative versions?)
Answer - because in Spanish there is no sound for "sh" :)
Spain was under Arabic influence for a quite a long and played as bridge translating Arabic knowledge to Europe. In Arabic there was some special word for unknowns similar to English "something", but it contained sounds "sh" which was not present in Spanish. It was somehow substituted by "K" and later under further translation to Latin it became "X"...