The Collaborative International Dictionary of English v.0.48:

Betwixt \Be*twixt"\, prep. [OE. betwix, bitwix, rarely bitwixt,
   AS. betweox, betweohs, betweoh, betw[imac]h; pref. be- by + a
   form fr. AS. tw[=a] two. See Between.]
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   1. In the space which separates; between.
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            From betwixt two aged oaks.           --Milton.
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   2. From one to another of; mutually affecting.
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            There was some speech of marriage
            Betwixt myself and her.               --Shak.
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   Betwixt and between, in a midway position; so-so; neither
      one thing nor the other. [Colloq.]
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WordNet (r) 3.0 (2006):

betwixt
    adv 1: in the interval; "dancing all the dances with little rest
           between" [syn: between, betwixt]