Fascinating article on the origin of "X" as "close". Interestingly, it appears as though the use of "X" is of Japanese origin, which comes as no great surprise.
"Batsu (x) is the symbol for incorrect, and can represent false, bad, wrong or attack, while maru (o) means correct, true, good, whole, or something precious. Batsu and maru are also common hand gestures. Cross your arms over your chest for batsu, circle your arms over your head for maru."
Read the full article here.