hill
岵 centers on a hill, specifically a hill covered with vegetation or trees.
岵 combines 山 (mountain) and 古 (old), likely indicating an old, tree-covered hill. The exact historical development is uncertain, but the component choice suggests a hill with established vegetation.
The left side is 山 (mountain), and the right side is 古 (old). Picture an old, weathered hill covered in ancient trees.
For コ, imagine a small child (子, こ) playing on the old hill: 子 -> コ, and the child runs up the tree-covered slope.