To shan someone is to sexually harass a person in a social setting such as in a club, a bar, a house party, ... in an unashamed and perverted fashion. Often done by virgins and closet gays as a way to appear as "playas" in front of their peers.
Dude I went clubbing with this douchebagthe other day, and he just totally attempted to shan some girl. She ran away crying and he was like "bro she's totally lesbo, she's lucky I even wanted to shan her".