The Collaborative International Dictionary of English v.0.48:

Twelve \Twelve\, a. [OE. twelve, twelf, AS. twelf; akin to
   OFries. twelf, twelef, twilif, OS. twelif, D. twaalf, G.
   zw["o]lf, OHG. zwelif, Icel. t[=o]lf, Sw. tolf, Dan. tolv,
   Goth. twalif, from the root of E. two + the same element as
   in the second part of E. eleven. See Two, and Eleven.]
   One more that eleven; two and ten; twice six; a dozen.
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   Twelve-men's morris. See the Note under Morris.

   Twelve Tables. (Rom. Antiq.) See under Table.
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