deafness
耾 centers on deafness, the inability to hear. The given meanings are limited to this single concept.
耾 combines 耳 (ear) with 厷, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to hearing or its loss.
The ear 耳 on the left is paired with a right side that looks like a blocked passage. Imagine the ear being sealed off, leading to deafness.
For コウ, picture a person cupping their ear and shouting 'Koh!' but hearing nothing: Koh -> コウ, and the ear remains deaf.