This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 肎 is uncertain. It appears to combine 冖 and 月, but the original component relationship is unclear.
With no clear meaning, picture a covered moon 月 under a lid 冖 as a mysterious, undefined object.
For コウ, imagine a covered moon glowing with a soft koh sound: koh -> コウ, like a quiet hum from the hidden moon.