The Collaborative International Dictionary of English v.0.48:

Dimidiate \Di*mid"i*ate\, v. t. [imp. & p. p. Dimidiated; p.
   pr. & vb. n. Dimidiating.]
   1. To divide into two equal parts. [Obs.] --Cockeram.
      [1913 Webster]

   2. (Her.) To represent the half of; to halve.
      [1913 Webster]