This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
襂 combines the clothing radical 衤 with the phonetic component 參, suggesting a type of garment. The exact historical development is uncertain.
The clothing radical 衤 hints at a garment, but the overall meaning is unclear. Picture a mysterious robe with the complex 參 pattern on it.
For シン, imagine a thin, sheer garment: thin -> シン, and the robe is so thin you can see the 參 pattern through it.