Northern California slang word for a helmet, often of the old military "steel pot" variety. It's worn by the neo-Nazis in the area.
Neo-Nazis and other wing nut types sometimes congregate and hold rallies in NoCal, or the Pacific Northwestern states to promote their hateful agenda. Some of them wear Klan Klown Konehead sheets, some wear brain buckets, some wear camies, some wear swastikas or other fascist insignia. It's funny that the helmets are called "brain buckets" because there's no brains in the thick heads underneath them, just piles of shit!