The quality or state of being stupid, stubborn, or lacking intelligence.
From 'boneheaded' (adjective) + '-ness' (abstract noun suffix), creating a noun that describes the quality of stupidity or stubborn behavior.
This word is a perfect example of how English builds abstract concepts—take an insult ('bonehead'), add an adjective suffix ('-ed'), then add '-ness' to make it philosophical, and suddenly you're discussing the very nature of stupidity!
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