The Collaborative International Dictionary of English v.0.48:

Certain \Cer"tain\, a. [F. certain, fr. (assumed) LL. certanus,
   fr. L. certus determined, fixed, certain, orig. p. p. of
   cernere to perceive, decide, determine; akin to Gr. ? to
   decide, separate, and to E. concern, critic, crime, riddle a
   sieve, rinse, v.]
   1. Assured in mind; having no doubts; free from suspicions
      concerning.
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            To make her certain of the sad event. --Dryden.
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            I myself am certain of you.           --Wyclif.
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   2. Determined; resolved; -- used with an infinitive.
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            However, I with thee have fixed my lot,
            Certain to undergo like doom.         --Milton.
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   3. Not to be doubted or denied; established as a fact.
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            The dream is certain, and the interpretation thereof
            sure.                                 --Dan. ii. 45.
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   4. Actually existing; sure to happen; inevitable.
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            Virtue that directs our ways
            Through certain dangers to uncertain praise.
                                                  --Dryden.
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            Death, as the Psalmist saith, is certain to all.
                                                  --Shak.
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   5. Unfailing; infallible.
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            I have often wished that I knew as certain a remedy
            for any other distemper.              --Mead.
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   6. Fixed or stated; regular; determinate.
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            The people go out and gather a certain rate every
            day.                                  --Ex. xvi. 4.
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   7. Not specifically named; indeterminate; indefinite; one or
      some; -- sometimes used independenty as a noun, and
      meaning certain persons.
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            It came to pass when he was in a certain city.
                                                  --Luke. v. 12.
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            About everything he wrote there was a certain
            natural grace und decorum.            --Macaulay.
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   For certain, assuredly.

   Of a certain, certainly.

   Syn: Bound; sure; true; undeniable; unquestionable;
        undoubted; plain; indubitable; indisputable;
        incontrovertible; unhesitating; undoubting; fixed;
        stated.
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WordNet (r) 3.0 (2006):

for certain
    adv 1: definitely or positively (`sure' is sometimes used
           informally for `surely'); "the results are surely
           encouraging"; "she certainly is a hard worker"; "it's
           going to be a good day for sure"; "they are coming, for
           certain"; "they thought he had been killed sure enough";
           "he'll win sure as shooting"; "they sure smell good";
           "sure he'll come" [syn: surely, certainly, sure,
           for sure, for certain, sure enough, sure as
           shooting]