Piss-stain graffiti is graffiti that is sprayed on someone else's property. It always lacks creativity and is sprayed for the purpose to look cool in front of your friends. It won't make you look cool, but it will make you an asshole.
Poser: Hey dude, check out what I tagged last night!
Normal kid: You fuck-up that's piss-stain graffiti!
The act of having anal intercourse with a woman reverse cowgirl style. Unknown to you she has severe diarrhea. As soon as you reach you cumming point she begins to release her bowels sliding off your penis and firmly sliding her ass all the way up to your face leaving a soggy strip of feces resembling a skid mark on your body. She then takes your hair and wipes her anus with it.
Girl: you wanna try anal for your birthday Guy: sure lets do it
Girl: lets go
Girl: (whispers to herself "you're getting a Mississippi piss stain. Happy birthday loser")