Definition of dunderheaded in US English:

dunderheaded

adjective

informal
  • See dunderhead

    • ‘Or is it another stupid, idiotic, no-brained, dunderheaded word game?’
    • ‘Their dunderheaded stupidity itself isn't enough to protect them, because they know when people like them and when they don't.’
    • ‘Now this, this was just a dunderheaded blunder.’
    • ‘What makes the dunderheaded dolt think he can pull it off?’
    • ‘There are all kinds of principled arguments against this dunderheaded and dangerous idea.’