bald as a coot
Totally bald. My father had long hair as a teen, but now he's bald as a coot.
as bald as a coot
completely bald. The coot (Fulica atra) has a broad white shield extending up from the base of its bill. The history of the word bald is somewhat obscure, but analogies with other northern European languages suggest a connection with the idea of ‘having a white patch or streak’.