It means Forever 21 in girl terms.
Girl: Hey! Wanna go to XXI with me?
Girl 2: Sure! I need new clothes anyways.
XXI is a badass gaming group. Don't mess with them or you'll be pwnd silly. The XXI's specialize in wifi games such as Super Smash Bros. Brawl, Pokemon Diamond/Pearl, and many others.
"Don't mess with XXI or you'll be pwn'd silly."