chap, crack, callus
胝 centers on hardened, roughened skin: a callus, a chap, or a crack that forms from repeated friction or dryness.
胝 combines the flesh component ⺼ with 氐, which likely contributes the sound. The character is associated with skin conditions like calluses and chapping.
The flesh component ⺼ on the left suggests skin, and the right side 氐 looks like a rough, cracked surface. Together they show skin that has become callused and chapped.
For チ, imagine a callus so thick it feels like a hard piece of cheese: cheese -> チ, and the rough skin on your hand matches the callus meaning.