This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
軱 combines 車 (vehicle) and 瓜 (melon). The exact historical development is uncertain, but it likely originally referred to a part of a cart or wheel, possibly a large bone-shaped component, given the kun reading おおぼね (large bone).
A cart (車) with a melon (瓜) inside might suggest a large, round object, like a big bone (おおぼね) used as a wheel part.
For コ, imagine a cart carrying a giant melon that is so heavy it makes a low 'ko' sound as it rolls: ko -> コ.