abbreviation for Young Seniors. These young people usually come in groups of friends who'd rather party together than with others. They call themselves Young Seniors because their main activities typically involve them lying about their ages. They can usually be found at your average slumber party, or at a dirty fraternity on a friday night.
person 1: hey what are we doing tonight?
person 2: yo, probably gonna YS this shit or something, we wanna have fun.
person 1: awhh yeahhh weed
bitch, i got mahh fake
person 2: yeah dude, but it's a school night so dont get too fucked up