This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The modern form combines 谷 (valley) and 空 (empty/sky), but the original semantic and phonetic roles are unclear.
With 谷 (valley) on the left and 空 (empty) on the right, picture a vast empty valley stretching into the distance, suggesting hollowness or emptiness.
For コウ, imagine the empty valley echoing with a hollow sound: koh -> コウ, like the sound of emptiness itself.