A “wángbādàn 忘/王八蛋” is the offspring of a woman lacking virtue. Another meaning of 王八 is 鼈 biē, fresh-water turtle. Turtle heads reemerging from hiding in the turtle’s shell look like the glans emerging from the foreskin, and turtles lay eggs. So a “wang ba” is a woman who has lost her virtue, and a “wang ba dan” is the progeny of such a woman, a turtle product, but, figuratively, also a penis product.
This profanity term has actually been widely used in China for many years, and it is a pretty offensive one to use. I find in both western and eastern culture, it is considered to be very offensive one when the subject is related to close family members.