pustules of any kind, rash or eruption, measles, various kinds of fever
胗 centers on skin eruptions and febrile illnesses: pustules, rashes, measles, and various fevers. The common thread is a visible, often raised, skin condition accompanied by heat or systemic illness.
胗 combines the flesh radical ⺼ (body part) with 㐱, which likely contributes a phonetic element. The character is associated with skin ailments, but the exact historical development is uncertain.
The flesh radical ⺼ indicates a body part, and 㐱 looks like a person with spots. Imagine a body covered in a rash of pustules: that's 胗.
For シン, picture a person with a rash on their shin: shin -> シン, and the rash is right on the flesh.