The Collaborative International Dictionary of English v.0.48:

Wag \Wag\, v. i.
   1. To move one way and the other; to be shaken to and fro; to
      vibrate.
      [1913 Webster]

            The resty sieve wagged ne'er the more. --Dryden.
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   2. To be in action or motion; to move; to get along; to
      progress; to stir. [Colloq.]
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            "Thus we may see," quoth he, "how the world wags."
                                                  --Shak.
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   3. To go; to depart; to pack oft. [R.]
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            I will provoke him to 't, or let him wag. --Shak.
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The Collaborative International Dictionary of English v.0.48:

Wag \Wag\, n. [From Wag, v.]
   [1913 Webster]
   1. The act of wagging; a shake; as, a wag of the head.
      [Colloq.]
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   2. [Perhaps shortened from wag-halter a rogue.] A man full of
      sport and humor; a ludicrous fellow; a humorist; a wit; a
      joker.
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            We wink at wags when they offend.     --Dryden.
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            A counselor never pleaded without a piece of pack
            thread in his hand, which he used to twist about a
            finger all the while he was speaking; the wags used
            to call it the thread of his discourse. --Addison.
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The Collaborative International Dictionary of English v.0.48:

Wag \Wag\, v. t. [imp. & p. p. Wagged; p. pr. & vb. n.
   Wagging.] [OE. waggen; probably of Scand. origin; cf. Sw.
   vagga to rock a cradle, vagga cradle, Icel. vagga, Dan.
   vugge; akin to AS. wagian to move, wag, wegan to bear, carry,
   G. & D. bewegen to move, and E. weigh. [root]136. See
   Weigh.]
   To move one way and the other with quick turns; to shake to
   and fro; to move vibratingly; to cause to vibrate, as a part
   of the body; as, to wag the head.
   [1913 Webster]

         No discerner durst wag his tongue in censure. --Shak.
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         Every one that passeth thereby shall be astonished, and
         wag his head.                            --Jer. xviii.
                                                  16.
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   Note: Wag expresses specifically the motion of the head and
         body used in buffoonery, mirth, derision, sport, and
         mockery.
         [1913 Webster]

WordNet (r) 3.0 (2006):

wag
    n 1: a witty amusing person who makes jokes [syn: wag, wit,
         card]
    2: causing to move repeatedly from side to side [syn: wag,
       waggle, shake]
    v 1: move from side to side; "The happy dog wagged his tail"
         [syn: wag, waggle]