The originaldefinition from the late 1980's characterizes those late teen-age and early twenty-something males with a prematurely rising/receding hairline.
Dude, what happened to you last summer? You went from a forehead to a five-head.
An exceptionally large forehead of which you can look into and see your own future. Often times light reflecting upon it will blind Pilots.
"Wow FUCK, uhhh that cunt Rachel has the biggest five-head I have ever seen, it took down a 747 earlier today killing 150 passengers, 4 of which were infants.