hill with trees or grass (some give the opposite meaning)
屺 refers to a hill covered with trees or grass, though some sources give the opposite meaning of a bare hill. The core idea is a small mountain with vegetation.
屺 combines 山 (mountain) with 己, which likely serves as a phonetic component. The character is rare and its exact historical development is uncertain.
The left side is 山 (mountain), and the right side 己 looks like a winding path. Picture a hill with a winding path covered in trees and grass.
For キ, imagine a key-shaped tree on the grassy hill: key -> キ, and the key unlocks the hill's secret.
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