shoulder bone
髃 centers on a specific bone: the shoulder bone. It is a precise anatomical term with no broader conceptual extensions.
髃 combines 骨 (bone) with 禺, which likely contributes the sound. The character is a phono-semantic compound for a specific bone.
The left side 骨 means bone, and the right side 禺 looks like a shoulder joint with its rounded top and lower structure. Picture a bone labeled with the shoulder joint shape.
For グ, imagine a shoulder bone that makes a gooey sound when you move it: goo -> グ, and the bone is the shoulder bone.