Tommy Hilfiger Black Button Up

I have only worn this one time! I have no use for it anymore, it is an XL. Completely honest here, I have no idea what the stain is on the back of the shirt. I wore it once to a wedding and the weather wasn't the greatest so maybe it had to do with that? Since then it's been sitting in my closet, I've taken it out to look at it and put it back but I've only worn it once. Other than the stain on the back that I genuinely do not know where it's from, it's wrinkled from being in a closet for years and also has a lot of lint and fuzzies from, once again, bring in a closet for years.