How did X come to mean "The UNKNOWN"?

X came to mean "the UNKNOWN" because people used it to talk about things they couldn’t see or figure out, like a mystery hiding behind a door.

Imagine you're playing hide and seek. You peek through the door, but you can't see who's hiding, that person is the UNKNOWN. Now imagine your friend says, “It’s someone with an X on their shirt.” That means you don’t know who it is, just that they have an X, and that makes them a mystery.

People started using X to stand for something hidden or not yet known, like the treasure map in your backpack. When you see an X on a map, you think, “There’s something special there, but I don’t know what it is.” That’s how X became the symbol for the UNKNOWN.

Think of X like a mystery box. You can't see what's inside, but you know something is in there. That’s exactly what X does: it says, “Something is here, but I haven’t found out what yet.”