出 looks like two 山 characters piled on top of each other, but in fact its origin has nothing to do with mountains. Many historical forms of this character, especially oracle bone script, show a footprint going out of some sort of enclosure: a room, a house, a city — anything with walls. On some of the oracle bone characters, the “enclosure” part even looks like an archaic form of 口 (that kind of toothy smile). Ponte Ryūrui mentions (and rejects) an alternative theory, where 出 is “based on an image of rapidly growing grass”.
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