The Collaborative International Dictionary of English v.0.48:

Decoct \De*coct"\, v. t. [imp. & p. p. Decocted; p. pr. & vb.
   n. Decocting.] [L. decoctus, p. p. of decoquere to boil
   down; de- + coquere to cook, boil. See Cook to decoct.]
   1. To prepare by boiling; to digest in hot or boiling water;
      to extract the strength or flavor of by boiling; to make
      an infusion of.
      [1913 Webster]

   2. To prepare by the heat of the stomach for assimilation; to
      digest; to concoct.
      [1913 Webster]

   3. To warm, strengthen, or invigorate, as if by boiling. [R.]
      "Decoct their cold blood." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decoct
    v 1: extract the essence of something by boiling it
    2: be cooked until very little liquid is left; "The sauce should
       reduce to one cup" [syn: boil down, reduce, decoct,
       concentrate]
    3: steep in hot water