moan, groan
呻 centers on vocal expressions of pain or discomfort: moaning and groaning.
呻 combines 口 (mouth) with 申, which likely contributes the sound and may suggest extending or stretching out a sound, fitting the idea of a drawn-out moan.
A mouth 口 making an extended sound 申: imagine a long, drawn-out groan coming from an open mouth.
For シン, picture a person moaning in pain while holding their shin: shin -> シン, and the groan comes from the mouth.