/əbˈtjuːs/
[adjective] Used to describe either an angle greater than 90 degrees but less than 180 degrees, or a person who is slow to understand things or who fails to see what should be obvious.
Calling someone 'obtuse' is literally saying their mind is blunt — too dull to cut through to the point.
The word 'obtuse' comes from the Latin word 'obtusus', which is the past participle of the verb 'obtundere', meaning 'to blunt' or 'to dull by striking'. This verb is built from two parts: 'ob-', a prefix meaning 'against', and 'tundere', meaning 'to strike' or 'to beat'. The original image is wonderfully physical — something being hammered repeatedly until its sharp edge is worn away and it becomes dull and rounded.
In classical Latin, 'obtusus' was used to describe physical objects that lacked a sharp point — a blunt sword, a rounded stone, anything that had been beaten into dullness. The Romans also used it to describe sounds that were muffled or not crisp, showing how the central idea of 'lacking sharpness' could apply to many senses at once. This reveals how deeply the Romans connected physical sharpness with clarity and effectiveness.
The word entered English in the 15th century, arriving through the educated channels of Latin scholarship rather than through everyday French borrowing. Its first home in English was in mathematics and geometry, where it described an angle that was 'blunt' — that is, wider and more rounded than a right angle, without the precise sharpness of something smaller. This geometric use kept the original Latin meaning almost perfectly intact, simply applying it to shape rather than to a physical object.
The leap from geometry to human intelligence happened gradually but logically. If a sharp angle was associated with precision and quickness, then a blunt or obtuse angle naturally suggested the opposite — something slow, rounded, and lacking in edge. By the 16th and 17th centuries, writers were freely calling people 'obtuse' to mean intellectually dull, slow to catch on, or stubbornly failing to understand something obvious. The insult lands with the same force today.
What makes 'obtuse' particularly interesting is how it connects to a whole family of English words through the root 'tundere'. The verb gave us 'contuse' (to bruise by beating) and 'contusion' (a bruise), travelling through Latin 'contusio'. It also connects to 'stun' — though that arrived through a more winding Old French route. The related Latin word 'obtundo' gave early medical writers a word for the dulling of pain or sensation, so doctors and geometers and philosophers were all drawing from the same well of meaning: the idea that repeated force creates dullness, and that dullness — whether of blade, angle, or mind — is the opposite of sharpness, clarity, and precision.