Making clumsy mistakes or moving clumsily without care; or describing someone who acts carelessly.
From Middle English 'blunderen,' possibly from Old Norse 'blundra' meaning 'to shut one's eyes' or 'be confused.' The sense of careless, blind action has remained throughout its history.
The word 'blunder' might come from the idea of acting with your eyes shut—it's like your brain isn't even looking at what you're doing, which is exactly what happens when someone makes a careless mistake!
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