crotch of a tree
椏 centers on the forked point where a tree branch splits from the trunk, the crotch of a tree.
椏 combines 木 (tree) with 亞, which likely contributes the sound and the idea of branching or forking. The exact historical development is uncertain.
A tree 木 with a branching 亞 shape on the right: picture the forked crotch where a limb splits from the trunk.
For また, imagine a tree crotch that looks like a pair of legs (また, crotch/groin in Japanese): the same word また connects the tree fork to the body fork.
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