The Collaborative International Dictionary of English v.0.48:

Thank \Thank\, v. t. [imp. & p. p. Thanked; p. pr. & vb. n.
   Thanking.] [AS. [thorn]ancian. See Thank, n.]
   To express gratitude to (anyone) for a favor; to make
   acknowledgments to (anyone) for kindness bestowed; -- used
   also ironically for blame.
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         "Graunt mercy, lord, that thank I you," quod she.
                                                  --Chaucer.
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         I thank thee for thine honest care.      --Shak.
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         Weigh the danger with the doubtful bliss,
         And thank yourself if aught should fall amiss.
                                                  --Dryden.
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The Collaborative International Dictionary of English v.0.48:

thank \thank\ (th[a^][ng]k), n.; pl. thanks (th[a^][ng]ks).
   [AS. [thorn]anc, [thorn]onc, thanks, favor, thought; akin to
   OS. thank favor, pleasure, thanks, D. & G. dank thanks, Icel.
   [thorn]["o]kk, Dan. tak, Sw. tack, Goth. [thorn]agks thanks;
   -- originally, a thought, a thinking. See Think.]
   A expression of gratitude; an acknowledgment expressive of a
   sense of favor or kindness received; obligation, claim, or
   desert, or gratitude; -- now generally used in the plural.
   "This ceremonial thanks." --Massinger.
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         If ye do good to them which do good to you, what thank
         have ye? for sinners also do even the same. --Luke vi.
                                                  33.
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         What great thank, then, if any man, reputed wise and
         constant, will neither do, nor permit others under his
         charge to do, that which he approves not, especially in
         matter of sin?                           --Milton.
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         Thanks, thanks to thee, most worthy friend,
         For the lesson thou hast taught.         --Longfellow.
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   His thanks, Her thanks, etc., of his or her own accord;
      with his or her good will; voluntary. [Obs.]
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            Full sooth is said that love ne lordship,
            Will not, his thanks, have no fellowship. --Chaucer.
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   In thank, with thanks or thankfulness. [Obs.]

   Thank offering, an offering made as an expression of
      thanks.
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WordNet (r) 3.0 (2006):

thank
    v 1: express gratitude or show appreciation to [syn: thank,
         give thanks]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "thank":
   acknowledge, appreciation, because of, blame, bless, by reason of,
   credit, due to, give credit, give thanks, gratefulness, gratitude,
   make acknowledgments of, offer thanks, owing to, recognition,
   recognize, render credit, render thanks, return thanks, thanks,
   thanks to, thanksgiving, through