the skull, the forehead
髗 centers on the bony structure of the head: the skull, and by extension the forehead as a prominent part of it.
髗 combines 骨 (bone) with 盧, which likely contributes the sound ロ and may have originally depicted a vessel or container, here metaphorically the bony container of the head.
The bone radical 骨 on the left tells you this is about bones. The right side 盧 looks like a complex container with a lid and base. Picture the skull as the bony container that holds the brain, with the forehead as its front plate.
For ロ, imagine a low, round skull rolling on the ground: low -> ロ, and the skull is the bony container of the head.