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The angle marked β is obtuse (sense 4) as it is greater than 90° but less than 180°

From Middle French obtus (obtuse (geometry); narrow-minded, obtuse; boring, dull, lifeless), from Latin obtūsus (blunt, dull; obtuse), past participle of obtundere, from obtundō (to batter, beat, strike; to blunt, dull), from ob- (prefix meaning against) + tundō (to beat, strike; to bruise, crush, pound) (ultimately from Proto-Indo-European *(s)tewd-, from *(s)tew- (to hit; to push)).



obtuse (comparative obtuser or more obtuse, superlative obtusest or most obtuse)

  1. Intellectually dull or dim-witted.
    • 2017 March 27, “The Observer view on triggering article 50: As Britain hurtles towards the precipice, truth and democracy are in short supply [editorial]”, in The Observer[1], London, archived from the original on 30 August 2017:
      Be you a Remainer or a Leaver, you would have to be particularly obtuse not to see that [Theresa] May's hard Tory Brexit will cost this country and its families more than it can conceivably afford.
  2. Indirect or circuitous.
  3. Of sound: deadened or muffled.
  4. (geometry) Of an angle: greater than 90 degrees but less than 180 degrees.
    • 1877, Ch[arles] Couche; James N. Shoolbred, transl., “Special Points in the Permanent Way”, in Permanent Way Rolling Stock and Technical Working of Railways. Followed by an Appendix on Works of Art, volume I, London: Dulau & Co., 37, Soho Square; Paris: Dunod, 49, Quai des Augustins, OCLC 58932800, § XIV (Rail-crossings), paragraph 258, page 316:
      Obtuse angles of the through crossing. — The system of the two obtuse-angled points is especially termed the dead-crossing. [] The point itself, less liable to damage than that of the crossing proper, on account of its obtuse form and its position relatively to the wheels, acts the same part towards the tapered portion of the cut rail, as the wing-rail does with respect to the acute-angle of the crossing.
    • 1922, May Sinclair, “Space, Time and Other Consciousnesses”, in The New Idealism, New York, N.Y.: The Macmillan Company, OCLC 558287, section v, pages 255–256:
      If he is standing close beside me I know that our separate axes of vision will meet at an acute angle in the centre of his object, and if we are further apart, at an obtuser angle.
  5. (geometry) Of a triangle: having one obtuse angle.
  6. (now chiefly botany, zoology) Not sharp; blunt.



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  1. feminine singular of obtus





  1. vocative masculine singular of obtūsus