Source : Webster's Revised Unabridged Dictionary (1913)

Uncertain \Un*cer"tain\, a. [Pref. un- + certain. Cf.
   {Incertain}.]
   1. Not certain; not having certain knowledge; not assured in
      mind; distrustful. --Chaucer.

            Man, without the protection of a superior Being, . .
            . is uncertain of everything that he hopes for.
                                                  --Tillotson.

   2. Irresolute; inconsonant; variable; untrustworthy; as, an
      uncertain person; an uncertain breeze.

            O woman! in our hours of ease, Uncertain, coy, and
            hard to please!                       --Sir W.
                                                  Scott.

   3. Questionable; equivocal; indefinite; problematical. ``The
      fashion of uncertain evils.'' --Milton.

            From certain dangers to uncertain praise. --Dryden.

   4. Not sure; liable to fall or err; fallible.

            Soon bent his bow, uncertain in his aim. --Dryden.

            Whistling slings dismissed the uncertain stone.
                                                  --Gay.

   Syn: See {Precarious}.

Uncertain \Un*cer"tain\, v. t. [1st pref. un- + certain; or fr.
   uncertain, a.]
   To make uncertain. [Obs.] --Sir W. Raleigh.

Source : WordNet®

uncertain
     adj 1: lacking or indicating lack of confidence or assurance;
            "uncertain of his convictions"; "unsure of himself and
            his future"; "moving with uncertain (or unsure)
            steps"; "an uncertain smile"; "touched the ornaments
            with uncertain fingers" [syn: {unsure}, {incertain}]
            [ant: {certain(p)}, {certain(p)}]
     2: not established beyond doubt; still undecided or unknown;
        "an uncertain future"; "a manuscript of uncertain origin";
        "plans are still uncertain"; "changes of great if
        uncertain consquences"; "without further evidence his
        story must remain uncertain" [ant: {certain(p)}]
     3: not established or confirmed; "his doom is as yet unsealed"
        [syn: {unsealed}] [ant: {sealed}]
     4: not firm or secure; "balancing three boxes in an uncertain
        pile"
     5: not certain to occur; not inevitable; "everything is
        uncertain about the army"; "the issue is uncertain" [ant:
        {certain}]
     6: subject to change; "a changeable climate"; "the weather is
        uncertain"; "unsettled weather with rain and hail and
        sunshine coming one right after the other" [syn: {changeable},
         {unsettled}]
     7: not safe from danger or mishap; "faced an uncertain future";
        "an unsure existence" [syn: {unsure}]
     8: not consistent or dependable; "an uncertain recollection of
        events"; "a gun with a rather uncertain trigger"
     9: ambiguous (especially in the negative); "she spoke in no
        uncertain terms"