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.
"this wicked cool bitch flap is like, stupid fresh"
"mary has the biggest bitch flap i've ever seen!"
"she got be right in the eye with her bitch flap!"
"I went shopping and all I bought was deodorant for sarah's bitch flap"