As bald as a coot

Meaning

Completely bald.

Origin

Coots are water birds whose heads have the appearance of baldness. This doesn't refer to the lack of feathers on the bird's head, but to their white markings. Bald has several meanings, one of which is 'streaked or marked with white'. That's the meaning here, as in pie-bald.

The phrase is very old and is referred to in John Lydgate's Chronicle of Troy, 1430:

"And yet he was as balde as is a coote."

See other 'as xxx as yyy' phrases.