This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 芤 is uncertain. It visually combines the grass radical 艹 with 孔, but the original semantic or phonetic role of 孔 in this character is unclear.
The grass radical 艹 sits above 孔, which can suggest a hole or opening. Picture a plant growing through a hole, but remember that the meaning itself is uncertain.
For コウ, imagine a small hole (孔) in the grass (艹) that makes a hollow 'koh' sound when the wind blows: koh -> コウ.