An enlargement of either the labia minora or majora on one side of the external female genitalia creating a flap which can be used to cover and protect the delicate flange
. Possibly secured by some sort of zip or poppers.
Extended labia on both sides is called a double bitch flap.
Bitch flaps are a favourite of lesbians and those too cheap to buy a chastity belt.
Not to be confused with bitch tits