birthmark, mole
痣 centers on a visible mark on the skin present from birth or appearing later: a birthmark or a mole.
痣 combines 疒 (sickness/ailment) with 志, which likely provides the sound シ. The exact historical development is uncertain, but the structure suggests a skin condition or mark.
The sickness enclosure 疒 wraps around 志 (will, intention). Imagine a mark on the skin that seems to have a will of its own, staying put like a stubborn birthmark or mole.
For シ, picture a small birthmark shaped like the letter 'C': 'C' -> シ, and that mark is a mole.
birthmark; nevus