"Tramp Stamp" Tattoo

Why is a tattoo on the lower back called a tramp stamp?  I can't stand that label.  

I got a tattoo on my hip when I was 23 or 24 after getting out of a bad relationship and into a better one. It meant something to me. I thought I might get another after getting my weight back to where I want it and my thyroid disease managed correctly (those go together). The only spot I would want another, I think, is on my lower back.  I think it can be pretty, sexy, but still hidden when I want it to be. 

Why call it a tramp stamp?

I am 41 years old, been married almost 18 years, monogamous, and the number of men I have slept with can be counted on one hand.  I'm not a tramp. Yet, if I get the tattoo where I want it, it will be called a tramp stamp.  My stepdaughter is BIG on calling it that.  I looked it up online and found a site where mean actually talked about how if they see a tattoo there on a woman, they think she is easier.  Really? What does the location of a tattoo have to do with anything?