The Collaborative International Dictionary of English v.0.48:

Earnest \Ear"nest\ ([~e]r"n[e^]st), n. [AS. eornost, eornest;
   akin to OHG. ernust, G. ernst; cf. Icel. orrosta battle,
   perh. akin to Gr. 'orny`nai to excite, L. oriri to rise.]
   Seriousness; reality; fixed determination; eagerness;
   intentness.
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         Take heed that this jest do not one day turn to
         earnest.                                 --Sir P.
                                                  Sidney.
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         And given in earnest what I begged in jest. --Shak.
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   In earnest, serious; seriously; not in jest; earnestly.
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The Collaborative International Dictionary of English v.0.48:

Earnest \Ear"nest\, a.
   1. Ardent in the pursuit of an object; eager to obtain or do;
      zealous with sincerity; with hearty endeavor; heartfelt;
      fervent; hearty; -- used in a good sense; as, earnest
      prayers.
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            An earnest advocate to plead for him. --Shak.
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   2. Intent; fixed closely; as, earnest attention.
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   3. Serious; important. [Obs.]
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            They whom earnest lets do often hinder. --Hooker.

   Syn: Eager; warm; zealous; ardent; animated; importunate;
        fervent; sincere; serious; hearty; urgent. See Eager.
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The Collaborative International Dictionary of English v.0.48:

Earnest \Ear"nest\, v. t.
   To use in earnest. [R.]
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         To earnest them [our arms] with men.     --Pastor Fido
                                                  (1602).
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The Collaborative International Dictionary of English v.0.48:

Earnest \Ear"nest\, n. [Prob. corrupted fr. F. arrhes, L. arra,
   arrha, arrhabo, Gr. 'arrabw`n, of Semitic origin, cf. Heb.
   [=e]r[=a]v[=o]n; or perh. fr. W. ernes, akin to Gael. earlas,
   perh. fr. L. arra. Cf. Arles, Earles penny.]
   1. Something given, or a part paid beforehand, as a pledge;
      pledge; handsel; a token of what is to come.
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            Who hath also sealed us, and given the earnest of
            the Spirit in our hearts.             --2 Cor. i.
                                                  22.
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            And from his coffers
            Received the golden earnest of our death. --Shak.
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   2. (Law) Something of value given by the buyer to the seller,
      by way of token or pledge, to bind the bargain and prove
      the sale. --Kent. Ayliffe. Benjamin.
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   Earnest money (Law), money paid as earnest, to bind a
      bargain or to ratify and prove a sale.

   Syn: Earnest, Pledge.

   Usage: These words are here compared as used in their
          figurative sense. Earnest is not so strong as pledge.
          An earnest, like first fruits, gives assurance, or at
          least a high probability, that more is coming of the
          same kind; a pledge, like money deposited, affords
          security and ground of reliance for the future.
          Washington gave earnest of his talent as commander by
          saving his troops after Braddock's defeat; his
          fortitude and that of his soldiers during the winter
          at Valley Forge might rightly be considered a pledge
          of their ultimate triumph.
          [1913 Webster]

WordNet (r) 3.0 (2006):

earnest
    adj 1: characterized by a firm and humorless belief in the
           validity of your opinions; "both sides were deeply in
           earnest, even passionate"; "an entirely sincere and cruel
           tyrant"; "a film with a solemn social message" [syn:
           earnest, sincere, solemn]
    2: earnest; "one's dearest wish"; "devout wishes for their
       success"; "heartfelt condolences" [syn: dear, devout,
       earnest, heartfelt]
    3: not distracted by anything unrelated to the goal [syn:
       businesslike, earnest]
    n 1: something of value given by one person to another to bind a
         contract