steep, promontory
嵜 centers on a steep, jutting landform: a sharp mountain peak or a rocky promontory extending into the sea.
嵜 combines 山 (mountain) with 竒, which likely contributes the sound and the idea of something unusual or steep. The exact historical development is uncertain.
A mountain 山 with a strange, steep shape 竒: picture a jagged peak jutting out like a promontory.
For キ, imagine a steep key-shaped promontory: key -> キ, and the mountain peak looks like a giant key.