Stupid or careless mistakes, or to make such mistakes.
From Old Norse 'blunda' (to shut one's eyes, to sleep), which evolved in Middle English to mean moving clumsily or making foolish mistakes—the connection being that you can't see what you're doing.
The Norse origin 'blunda' (to shut your eyes) perfectly explains why a blunder is such a silly mistake—you're basically making an error while 'eyes shut'! The word's etymology captures exactly why we call careless mistakes 'blunders.'
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