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1.
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
by Neil Patrick Harriss September 09, 2012