A charlatan at being a doctor of any kind.
My psychologist wasn't a quack because she behaved like a doctor.
Money. Derived from duckets. The latter would often be shortened to "ducks", which eventually led to "quacks" (being the noise a duck makes).
"Yo, you snag that satchel?"
"Yea, yo. Shit was dough stack, though. I gotta get on making them quacks."