This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鱵 combines 魚 (fish) with 箴, which likely contributes the sound. The exact historical development is uncertain.
The left side 魚 means fish, and the right side 箴 looks like a needle or probe. Imagine a needle-like fish, perhaps a halfbeak or saury, though the exact meaning is unclear.
For シン, picture a thin, needle-like fish shimmering in the water: shin -> シン, like the glint of its slender body.