. . . to show unconscious or dead, that is.
So, I’m watching Christmas Story for the billionth time and when Ralphie shoots the bad guys, they’re lying in a heap with Xs over their eyelids. The sudden thought popped into my head, “Where did that meme originate, anyway?”
I’m guessing comic strips since with the simple drawing style, a character with closed eyes would look more asleep rather than comatose or worse.