corner, spire, goblet, winecup
柧 centers on a pointed or angular form: a corner, a spire, or a vessel like a goblet or winecup that tapers to a point.
柧 combines 木 (tree, wood) with 瓜 (melon, gourd). The exact historical development is uncertain, but it likely originally referred to a wooden object with a pointed or gourd-like shape, such as a corner piece or a cup.
A wooden (木) gourd (瓜) carved into a pointed shape: imagine a wooden goblet with a sharp corner or spire-like top.
For コ, picture a wooden goblet so small it's just a 'co'rner cup: co -> コ.