The Collaborative International Dictionary of English v.0.48:

Cress \Cress\ (kr[e^]s), n.; pl. Cresses (kr[e^]s"[e^]z). [OE.
   ces, cresse, kers, kerse, AS. cresse, cerse; akin to D. kers,
   G. kresse, Dan. karse, Sw. krasse, and possibly also to OHG.
   chresan to creep.] (Bot.)
   A plant of various species, chiefly cruciferous. The leaves
   have a moderately pungent taste, and are used as a salad and
   antiscorbutic.
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   Note: The garden cress, called also peppergrass, is the
         Lepidium sativum; the water cress is the Nasturtium
         officinale. Various other plants are sometimes called
         cresses.
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               To strip the brook with mantling cresses spread.
                                                  --Goldsmith.
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   Bitter cress. See under Bitter.

   Not worth a cress, or "not worth a kers." a common old
      proverb, now turned into the meaningless "not worth a
      curse." --Skeat.
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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):

bitter cress
    n 1: any of various herbs of the genus Cardamine, having usually
         pinnate leaves and racemes of white, pink or purple
         flowers; cosmopolitan except Antarctic [syn: bittercress,
         bitter cress]