Originally, a derogatory term for a dog too sorry to hunt anything but his own food. Can also be used for a person who is slow, lazy, or selfish.
An example, as from the movie it is based on (The Biscuit Eater), is a hunting dog that attacks the birds and eats them for himself instead of pointing or retrieving them.
The original meaning of the word Biscuit-eater comes from when a group of men masturbate together in a circle with a biscuit on the floor between them, whoever cums last on the biscuit then has to eat the biscuit.