hairpin, clasp, wear in hair
兂 centers on a hairpin or clasp used to secure hair. The meanings all relate to this small decorative or functional object worn in the hair.
The exact historical development of 兂 is uncertain. It may be a pictograph of a hairpin, but the modern form does not clearly preserve that image.
The top horizontal and diagonal strokes suggest the prongs of a hairpin, while the lower part resembles the decorative end. Imagine a simple two-pronged clasp sliding into hair.
For シン, picture a shiny hairpin: shin -> シン, and the clasp gleams as it holds the hair in place.