This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
岈 combines 山 (mountain) with 牙 (tusk, fang). The exact historical development is uncertain, and the character is rare in Japanese.
The left side is a mountain 山, and the right side is a tusk 牙. Picture a mountain shaped like a tusk, but its meaning remains unclear.
For コ, imagine a small child (子, こ) playing near the tusk-shaped mountain: 子 -> コ, and the mountain echoes the sound.