the leg of a boot
靿 refers specifically to the leg part of a boot, the section that covers the shin and calf.
靿 combines 革 (leather) with 幼, which likely contributes the sound. The character is a phono-semantic compound for a leather boot leg.
The left side 革 means leather, and the right side 幼 suggests something young or small. Picture a young child's leather boot, focusing on the leg part that wraps the calf.
For オウ, imagine the boot leg being worn by an old man who says 'Ow!' as he pulls it on: 'Ow' -> オウ, and the leather boot leg is the source of the discomfort.