This trope refers to the ability of hair, nails, etc. to survive things that there is no narrative justification for. Example: In the Terminator mythos, nothing dead should be able to time-travel outside of your living tissue. Yet everybody goes through just fine with a full head of hair. It's also common for supernatural beings such as vampires to be deeply and severely burnt, yet for their hair to grow back into the same length and style before they've even stopped lurking in sewers. When this trope is in effect, even tattoos will grow back perfectly.
Hello, Unknown Troper. You'll need to get known to lend a hand here.