The Collaborative International Dictionary of English v.0.48:

Rue \Rue\ (r[udd]), n. [F. rue, L. ruta, akin to Gr. "ryth`; cf.
   AS. r[=u]de.]
   1. (Bot.) A perennial suffrutescent plant (Ruta
      graveolens), having a strong, heavy odor and a bitter
      taste; herb of grace. It is used in medicine.
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            Then purged with euphrasy and rue
            The visual nerve, for he had much to see. --Milton.
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            They [the exorcists] are to try the devil by holy
            water, incense, sulphur, rue, which from thence, as
            we suppose, came to be called herb of grace. --Jer.
                                                  Taylor.
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   2. Fig.: Bitterness; disappointment; grief; regret.
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   Goat's rue. See under Goat.

   Rue anemone, a pretty springtime flower (Thalictrum
      anemonides) common in the United States.

   Wall rue, a little fern (Asplenium Ruta-muraria) common
      on walls in Europe.
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The Collaborative International Dictionary of English v.0.48:

Rue \Rue\, v. i.
   1. To have compassion. [Obs.]
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            God so wisly [i. e., truly] on my soul rue.
                                                  --Chaucer.
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            Which stirred men's hearts to rue upon them.
                                                  --Ridley.
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   2. To feel sorrow and regret; to repent.
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            Work by counsel and thou shalt not rue. --Chaucer.
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            Old year, we'll dearly rue for you.   --Tennyson.
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The Collaborative International Dictionary of English v.0.48:

Rue \Rue\, n. [AS. hre['o]w. See Rue, v. t.]
   Sorrow; repetance. [Obs.] --Shak.
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The Collaborative International Dictionary of English v.0.48:

Rue \Rue\, v. t. [imp. & p. p. Rued (r[udd]d); p. pr. & vb. n.
   Ruing.] [OE. rewen, reouwen, to grive, make sorry, AS.
   hre['o]wan; akin to OS. hrewan, D. rouwen, OHG. hriuwan, G.
   reuen, Icel. hryggr grieved, hryg[eth] sorrow. [root] 18. Cf.
   Ruth.]
   1. To lament; to regret extremely; to grieve for or over.
      --Chaucer.
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            I wept to see, and rued it from my heart. --Chapmen.
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            Thy will
            Chose freely what it now so justly rues. --Milton.
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   2. To cause to grieve; to afflict. [Obs.] "God wot, it rueth
      me." --Chaucer.
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   3. To repent of, and withdraw from, as a bargain; to get
      released from. [Prov. Eng.]
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WordNet (r) 3.0 (2006):

rue
    n 1: European strong-scented perennial herb with grey-green
         bitter-tasting leaves; an irritant similar to poison ivy
         [syn: rue, herb of grace, Ruta graveolens]
    2: leaves sometimes used for flavoring fruit or claret cup but
       should be used with great caution: can cause irritation like
       poison ivy
    3: sadness associated with some wrong done or some
       disappointment; "he drank to drown his sorrows"; "he wrote a
       note expressing his regret"; "to his rue, the error cost him
       the game" [syn: sorrow, regret, rue, ruefulness]
    4: (French) a street or road in France
    v 1: feel remorse for; feel sorry for; be contrite about [syn:
         repent, regret, rue]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "rue":
   affliction, anguish, be sorry for, bemoan, bewail, care,
   commiseration, compassion, compunction, contriteness, contrition,
   deplore, dole, grief, heartache, heartbreak, penance, penitently,
   regret, remorse, remorsefulness, repent, repentance, repine,
   rue the day, ruth, sympathy, woe



Easton's 1897 Bible Dictionary:

Rue
   a garden herb (Ruta graveolens) which the Pharisees were careful
   to tithe (Luke 11:42), neglecting weightier matters. It is
   omitted in the parallel passage of Matt. 23:23. There are
   several species growing wild in Palestine. It is used for
   medicinal and culinary purposes. It has a powerful scent, and is
   a stimulant. (See MINT.)