going away present, parting gift
贐 centers on a gift given to someone who is departing: a farewell present or parting gift.
贐 combines 貝 (shell, money) with 盡, which likely contributes the sound and the idea of something being used up or given fully. The character expresses a monetary or valuable gift given at parting.
The left side 貝 represents money or valuables, and the right side 盡 suggests something being exhausted or fully given. Picture a friend giving you a generous farewell gift of shells (money) as you depart, leaving them with nothing left to give.
For シン, imagine a shiny new coin given as a parting gift: shiny -> シン, and the coin is the farewell present.