They didn't start as a way to signify women that got around, but as any stereo type starts: you take a handful sample and come to a conclusion that applies to all.

You often saw the rough looking (booze and cigarettes taking its toll) party life ladies displaying the tattoo in everyday scenarios.

You occasionally saw somebody that looked cleancut have them too.

The rest were probably hidden by higher rise clothing.

So a stereotype starts, and a catchy rhyming term to go with it.