Right, so I'm late to this thread party, but I thought I'd add my pic and story anyway.
It's on my lower back but not as low as the 'tramp stamp' style. It's been retouched once. I got it when I was 17, well before the X-Men films came out in the cinema. It's about eight years old now.
When I was young, one of only good things about life was watching the X-Men cartoon that used to come on Fox with my brother. I read the comics and loved anything Marvel related. My favourite books were the Generation X series, and my favourite character was (and remains) Marrow.
*siiiigh* Isn't she beautiful?
Anyway, when the films came out, they summarily broke my heart because the comics almost instantly changed to match the films. I hadn't listened to my comic geek friends over the years very well... because they'd had their hearts broken when the Silver and Golden ages ended. Everything changes, everything evolves, but in Marvel, everything first has to make money.
I still love comics. I have an original painting of a group of X-Men on my wall. But the reason I got the tattoo was because I wanted something that meant something to me, reminded me of the innocence I used to have before cynicism took complete hold, and I wanted something that could have multiple meanings. X's and O's show up in antiquity all the time, in practically every prehistoric culture on the planet. And that works well too, because I'm a prehistoric archaeologist.