The Leath Hedger, also known as the reverse joker, or the smiling skull, is the surgical scar left after a hair transplant procedure known as a follicular unit extraction (FUE).
Timmy can never shave his head again because of the Leath Hedger scar at the back of his head. He looks like the joker from the new batman movie put a "smile on the back of his head"
by Mr M.L.G. June 17, 2009