The Collaborative International Dictionary of English v.0.48:

Bitter \Bit"ter\, a. [AS. biter; akin to Goth. baitrs, Icel.
   bitr, Dan., Sw., D., & G. bitter, OS. bittar, fr. root of E.
   bite. See Bite, v. t.]
   1. Having a peculiar, acrid, biting taste, like that of
      wormwood or an infusion of hops; as, a bitter medicine;
      bitter as aloes.
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   2. Causing pain or smart; piercing; painful; sharp; severe;
      as, a bitter cold day.
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   3. Causing, or fitted to cause, pain or distress to the mind;
      calamitous; poignant.
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            It is an evil thing and bitter, that thou hast
            forsaken the Lord thy God.            --Jer. ii. 19.
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   4. Characterized by sharpness, severity, or cruelty; harsh;
      stern; virulent; as, bitter reproach.
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            Husbands, love your wives, and be not bitter against
            them.                                 --Col. iii.
                                                  19.
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   5. Mournful; sad; distressing; painful; pitiable.
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            The Egyptians . . . made their lives bitter with
            hard bondage.                         --Ex. i. 14.
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   Bitter apple, Bitter cucumber, Bitter gourd. (Bot.) See
      Colocynth.

   Bitter cress (Bot.), a plant of the genus Cardamine, esp.
      Cardamine amara.

   Bitter earth (Min.), tale earth; calcined magnesia.

   Bitter principles (Chem.), a class of substances, extracted
      from vegetable products, having strong bitter taste but
      with no sharply defined chemical characteristics.

   Bitter salt, Epsom salts; magnesium sulphate.

   Bitter vetch (Bot.), a name given to two European
      leguminous herbs, Vicia Orobus and Ervum Ervilia.

   To the bitter end, to the last extremity, however
      calamitous.
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   Syn: Acrid; sharp; harsh; pungent; stinging; cutting; severe;
        acrimonious.
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WordNet (r) 3.0 (2006):

Vicia orobus
    n 1: European perennial toxic vetch [syn: bitter betch, Vicia
         orobus]