used in place names
This rare kanji has no clear standalone meaning. It is used in place names, so treat its meaning as uncertain.
垕 combines 后 (empress, after) and 土 (earth). The exact historical development is uncertain, but it likely originated as a place-name character.
The top 后 suggests a ruler or behind, and the bottom 土 is earth. Picture an ancient place name carved into the earth behind a palace.
For コウ, imagine a place name sign with the kanji 垕, and a traveler saying 'Koh!' when they finally find it: Koh -> コウ.