No 'folding back' is not a medical term. In older people the skin is very fragile. A tear in the skin can cause it to fold back. In the picture below, the flap of skin has a tenuous connection with the underlying tissues, and could easily fold back. Naturally, you'd need a deeper wound to have skin folding back off bones and ligaments. The text does say it was a "thick piece".
Skin does not normally fold back, but the context is given that a nurse is treating it, so we're talking about an injury.
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