A douchebag way of rubbing into people who aren't seniors. Stands for "You're Only a Senior Once". Using it too much will cause people (underclassmen) to hate you.
Junior: Oh shit nigga, I got soo much AP Bio HW, looks like I can't make G's awesome ass/thug party.
Senior: Hahaha sucks. #YOSO son. I don't gotta do shit nigga