to dress the hair
髽 centers on arranging or styling the hair, specifically dressing it in a particular way.
髽 combines 髟 (hair) with 坐 (sit), suggesting hair that is dressed or arranged, possibly in a style that sits on the head. The exact historical development is uncertain.
The top 髟 is hair, and the bottom 坐 looks like two people sitting. Imagine someone sitting down to have their hair carefully dressed and styled.
For サ, picture a stylist named Sa who dresses hair: Sa -> サ, and she is sitting (坐) arranging the hair (髟).