(as) bald as a billiard ball

Totally bald. Balls used in billiards are shiny and smooth. My father had long hair as a teen, but now he's bald as a billiard ball. The medicine they gave me to treat the disease left me bald as a billiard ball! If you take after my side of the family, you'll be as bald as a billiard ball by 45.

bald as a coot/billiard ball

Very bald indeed. The coot is a black waterbird whose white bill extends up to the forehead, making it appear to be bald. Indeed, this bird was already being called a balled cote in the thirteenth century. The later simile, to a billiard ball, has been less recorded, but since billiards was already popular in Shakespeare’s day it cannot be of very recent origin.