Why is a tattoo on the lower back of a woman referred to as a Tramp Stamp or California License Plate?

Answer 1

According to Wikipedia, an American woman's lower back tattoo became popular in the late 1990's among young women. Such a tattoo is commonly referred to as a tramp stamp. This object of personal expression and wearable art is often showcased by women wearing midriff's, low rise jeans and belly chains. It is also known as "bitch tat".

Though it is a stereotype, it is often speculated that women with such tattoos may be sexually promiscuous, hence the more derogatory names. Women who have these tattoos often tend to be rather confident types who are comfortable with their sexuality. Sadly, this intimidates some men who must make themselves feel better with name-calling.

Since such tattoos are most commonly found among women in California, they are sometimes called California License Plates.

"Yo, I can see that chick's California license plate sticking out from her low-rise jeans."