The Collaborative International Dictionary of English v.0.48:

Now \Now\ (nou), adv. [OE. nou, nu, AS. n[=u], nu; akin to D.,
   OS., & OHG. nu, G. nu, nun, Icel., n[=u], Dan., Sw., & Goth.
   nu, L. nunc, Gr. ny`, ny^n, Skr. nu, n[=u]. [root]193. Cf.
   New.]
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   1. At the present time; at this moment; at the time of
      speaking; instantly; as, I will write now.
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            I have a patient now living, at an advanced age, who
            discharged blood from his lungs thirty years ago.
                                                  --Arbuthnot.
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   2. Very lately; not long ago.
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            They that but now, for honor and for plate,
            Made the sea blush with blood, resign their hate.
                                                  --Waller.
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   3. At a time contemporaneous with something spoken of or
      contemplated; at a particular time referred to.
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            The ship was now in the midst of the sea. --Matt.
                                                  xiv. 24.
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   4. In present circumstances; things being as they are; --
      hence, used as a connective particle, to introduce an
      inference or an explanation.
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            How shall any man distinguish now betwixt a parasite
            and a man of honor?                   --L'Estrange.
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            Why should he live, now nature bankrupt is? --Shak.
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            Then cried they all again, saying, Not this man, but
            Barabbas. Now, Barabbas was a robber. --John xviii.
                                                  40.
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            The other great and undoing mischief which befalls
            men is, by their being misrepresented. Now, by
            calling evil good, a man is misrepresented to others
            in the way of slander.                --South.
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   Now and again, now and then; occasionally.

   Now and now, again and again; repeatedly. [Obs.] --Chaucer.

   Now and then, at one time and another; indefinitely;
      occasionally; not often; at intervals. "A mead here, there
      a heath, and now and then a wood." --Drayton.

   Now now, at this very instant; precisely now. [Obs.] "Why,
      even now now, at holding up of this finger, and before the
      turning down of this." --J. Webster (1607).

   Now . . . now, alternately; at one time . . . at another
      time. "Now high, now low, now master up, now miss."
      --Pope.
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WordNet (r) 3.0 (2006):

now now
    adv 1: interjection of rebuke