Not sharp or pointed; slow to understand things; lacking quickness of mind or perception.
From Latin 'obtusus' meaning 'beaten down' or 'blunt'. The geometric sense (an angle greater than 90 degrees) and the intellectual sense both derive from the basic meaning of being dull or blunt.
An obtuse angle is literally the 'dull' angle—more than 90 degrees—while an acute angle is 'sharp,' so geometry teachers are using thousand-year-old metaphors about bluntness vs. sharpness.
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