shoulder blade
胛 refers specifically to the shoulder blade, the flat bone at the back of the shoulder.
胛 combines the flesh radical ⺼ with 甲, which likely contributes the sound and may suggest a shell-like or flat shape, fitting the shoulder blade.
The flesh radical ⺼ indicates a body part, and 甲 looks like a flat, shell-like plate. Picture the shoulder blade as a flat shell of flesh on your back.
For コウ, imagine a coat (コート) hanging on your shoulder blade: coat -> コウ, and the shoulder blade supports it.