90951
Someone so old school Riverside, California that they have lived through both area codes, 909 and 951.
Also a reference to a new school "bro". A reference to basically every guy in Riverside, as distinguished by wearing sleeveless black jerseys, black skater hats, black shorts, lifted trucks with a grenade sticker on it.
That Bro is so "90951", he just got another grenade tattoo on his hand.