The Collaborative International Dictionary of English v.0.48:

Twitter \Twit"ter\, v. i. [imp. & p. p. Twittered; p. pr. &
   vb. n. Twittering.] [OE. twiteren; of imitative origin; cf.
   G. zwitschern, OHG. zwizzir[=o]n, D. kwetteren, Sw. qvittra,
   Dan. quiddre. Cf. Titter.]
   1. To make a succession of small, tremulous, intermitted
      noises.
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            The swallow twittering from the straw-built shed.
                                                  --Gray.
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   2. To make the sound of a half-suppressed laugh; to titter;
      to giggle. --J. Fletcher.
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   3. [Perhaps influenced by twitch.] To have a slight trembling
      of the nerves; to be excited or agitated.
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