GBW is an acronym for "get butt wednesdays." Essentially, every wednesday all after school commitments - whether it be academic or athletic - are put aside and getting butt is put as the top priority. The tradition was initiated by a kid in San Francisco named Zev Newman, known for always missing wednesday appointments.
Bro1 - "You hittin' up practice afterschool today?"
Bro2 - "Nah it's gbw breh"
Bro1 - "Oh I got you"
Bro2 - "Yuhhhh"