Slang. A term used for a rather large, flappy pair of labia majora which resemble a dangly cut of beef. (Think of two flapping sirloin steaks)
'Beef flaps' are usually dark red in colour, look slightly raw and bloddy and have a lot of veins and sinew running through them.
I saw your mum's beef flaps the other day, they hung out the side of her bikini, it was rather sick, especially as i'd had sirloin steak for dinner.
Dedicated to my lovely housemates, Percy, Lex and Kieron ;-)