Super mega ultra best friend. Usually used for freind zone or really close friends.
Friend#1: I wanna be more then friends
Friend#2: Best friends?
Friend#1: No something more
Friend#2: SMUBF
Friend #1:............
When you slap an add so hard it creates an echo heard throughout the room
Person #1: Did you hear that noise?
Person #2: Yeah that was Richard giving his bae a sonic smack