This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
茳 combines the grass radical 艹 with 江 (river). It likely originated as a plant name, possibly referring to a waterside plant, but the exact historical development is uncertain.
The top grass 艹 and bottom river 江 suggest a plant growing by the water. Picture a grassy riverbank plant whose name is now obscure.
For コウ, imagine a koi fish swimming in the river 江 beneath the grass 艹: koi -> コウ, and the plant sways by the water.
—