bridle
鞚 centers on a bridle, the leather headgear used to control a horse.
鞚 combines 革 (leather) with 空, which likely provides the sound コウ. The character denotes a leather bridle.
Leather (革) that is empty (空) of a horse is just a bridle waiting to be used.
For コウ, imagine a horse controlled by a leather bridle: the sound of the horse's hoofbeat is a steady 'koh, koh' (コウ).