type of tree, empty
桍 is a rare kanji with two disconnected meanings: a type of tree and emptiness. The tree sense likely comes from the wood radical, while the emptiness sense may be a phonetic loan or extension. Treat these as separate ideas without a single conceptual center.
桍 combines 木 (tree) with 夸, which may have contributed a phonetic value. The exact historical development is uncertain, and the connection to 'empty' is unclear.
A tree (木) with an exaggerated, empty trunk (夸 suggests exaggeration or emptiness). Imagine a hollow tree that looks impressive but is empty inside.
For コ, picture a hollow tree that echoes with a 'ko' sound when you knock on it: ko -> コ.