sad, revolve
軫 centers on a heavy, inward-turning motion: a cart revolving or a mood sinking into sadness. The two given meanings connect through the idea of something turning in on itself.
軫 combines 車 (cart, vehicle) with 㐱, which likely contributes the sound and may relate to turning or twisting. The exact historical development of 㐱 is uncertain, but the character is associated with the revolving of a cart and, by extension, a turning inward of emotion.
A cart (車) with a twisted, tangled right side (㐱) looks like its wheels are stuck in a sad, endless revolve. The cart can't move forward, so it just spins in place, heavy with sorrow.
For シン, imagine the cart revolving sadly in a deep, dark shin (シン) of a tree trunk, stuck and going nowhere. The shin of the wood traps the wheel, making it spin in place.