obtuse: [16] The etymological meaning of obtuse is ‘beaten down, blunted’. It comes from Latin obtūsus, the past participle of obtundere, a compound verb formed from the prefix ob- ‘against’ and tundere ‘beat’ (source of English contusion and related to toil). The notion of being ‘dulled’ or ‘blunted’ led to its being used for ‘having dulled wits, stupid’, and the idea of bluntness also lies behind its geometrical use for an angle of more than 90 degrees (as contrasted with the ‘sharp’ acute angle). => contusion, toil
obtuse (adj.)
early 15c., "dull, blunted," from Middle French obtus (fem. obtuse), from Latin obtusus "blunted, dull," also used figuratively, past participle of obtundere "to beat against, make dull," from ob "against" (see ob-) + tundere "to beat," from PIE *(s)tud-e- "to beat, strike, push, thrust," from root *(s)teu- "to push, stick, knock, beat" (cognates: Latin tudes "hammer," Sanskrit tudati "he thrusts"). Sense of "stupid" is first found c. 1500. Related: Obtusely; obtuseness.