Brain Bucket Meaning

Example : 1956 August 13, "Education: Pigs Aren't Pigs," Time:  The Air Force not only makes up words and phrases (e.g., brain bucket for crash helmet) . . . .1987 July 7, Susan Bickelhaupt, "Tour de France Packed Solid," Boston Globe, Sports p. 33:  Roy Knickman of 7-Eleven said he fears more crashes today and will not keep his helmet far from sight. "I always wear my brain bucket," he said.2003 July 6, Alex Berenson, "Ideas & Trends: Born to Be Wild, but at a Cost," New York Times (retrieved 1 Oct 2012):  Riding a 500-pound motorcycle in the middle of 3-ton Ford Expeditions and 40-ton trucks is dangerous, with or without a brain bucket or a leather jacket.1856, Frederick W. Saunders, "A Yarn in The Long Boat", Ballou's Monthly Magazine, vol. 4, no. 4, p. 326 (Google preview):  [H]e fetched me such a clip on top of my brain-bucket as to drive all my senses clear down into my boots.1880, Daniel Wise, Oscar's Boyhood; or, The Sailor's Son, ch. 6 (Google preview):  "[B]ut come, Ed, haven't you got your brain-bucket mended yet, so that you can tell us a story?"