black, glossy hair
鬒 centers on the appearance of thick, lustrous black hair. The given meanings describe this specific visual quality rather than separate concepts.
鬒 combines 髟 (hair) with 真, which likely contributes the sound and may reinforce the idea of true or genuine blackness. The exact historical development is uncertain.
The top 髟 is the hair radical, and the bottom 真 means truth or reality. Imagine hair so genuinely black and glossy that it looks like a perfect, true black.
For シン, picture a shiny black hairpin: shin -> シン, and the hairpin gleams against the glossy black hair.