This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
棝 combines 木 (tree/wood) with 固 (hard/solid), but the exact historical development and original meaning are uncertain.
The left side is wood 木, and the right side is solid 固. Picture a solid wooden device, perhaps a trap, but its exact meaning is uncertain.
For コ, imagine a solid wooden trap that makes a 'ko' sound when it snaps shut: ko -> コ.