Definition of "bubblehead"

bubblehead

noun
plural bubbleheads
1. (slang) A stupid person.
  Quotations
  I rather enjoyed it, but my niece said Audrey Hepburn was a “bubblehead”.
  2004, David Mitchell, Cloud Atlas, London: Hodder and Stoughton
2. (slang) A submariner; bubble-head.
3. (slang) A navy hard hat or salvage diver (inspired by the shape of the old spun-copper diving helmet).