The Collaborative International Dictionary of English v.0.48:

Sure \Sure\, a. [Compar. Surer; superl. Surest.] [OE. sur,
   OF. se["u]r, F. s[^u]r, L. securus; se aside, without + cura
   care. See Secure, and cf. Assure, Insure, Sicker
   sure.]
   1. Certainly knowing and believing; confident beyond doubt;
      implicity trusting; unquestioning; positive.
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            We are sure that the judgment of God is according to
            truth against them which commit such things. --Rom.
                                                  ii. 2.
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            I'm sure care 's an enemy of life.    --Shak.
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   2. Certain to find or retain; as, to be sure of game; to be
      sure of success; to be sure of life or health.
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   3. Fit or worthy to be depended on; certain not to fail or
      disappoint expectation; unfailing; strong; permanent;
      enduring. "His sure word." --Keble.
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            The Lord will certainly make my lord a sure house;
            because my lord fighteth the battles of the Lord.
                                                  --1 Sam. xxv.
                                                  28.
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            The testimony of the Lord is sure.    --Ps. xix. 7.
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            Which put in good sure leather sacks. --Chapman.
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   4. Betrothed; engaged to marry. [Obs.]
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            The king was sure to Dame Elizabeth Lucy, and her
            husband before God.                   --Sir T. More.
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            I presume . . . that you had been sure as fast as
            faith could bind you, man and wife.   --Brome.
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   5. Free from danger; safe; secure.
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            Fear not; the forest is not three leagues off;
            If we recover that we are sure enough. --Shak.
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   To be sure, or Be sure, certainly; without doubt; as,
      Shall you do? To be sure I shall.

   To make sure.
      (a) To make certain; to secure so that there can be no
          failure of the purpose or object. "Make Cato sure."
          --Addison. "A peace can not fail, provided we make
          sure of Spain." --Sir W. Temple.
      (b) To betroth. [Obs.]
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                She that's made sure to him she loves not well.
                                                  --Cotgrave.
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   Syn: Certain; unfailing; infallible; safe; firm; permanent;
        steady; stable; strong; secure; indisputable; confident;
        positive.
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WordNet (r) 3.0 (2006):

to be sure
    adv 1: admittedly; "to be sure, he is no Einstein" [syn: to be
           sure, without doubt, no doubt]