This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
宖 combines 宀 (roof) with 弘, but the historical development and original meaning are uncertain.
The roof 宀 over 弘 suggests a sheltered space, but the specific meaning is not established.
For コウ, imagine a calm koi pond under a roof: koi -> コウ, and the roof 宀 shelters the koi.