The Collaborative International Dictionary of English v.0.48:

Cacodylic \Cac`o*dyl"ic\, a. (Chem.)
   Of, pertaining to, or derived from, cacodyl.
   [1913 Webster]

   Cacodylic acid, a white, crystalline, deliquescent
      substance, (CH3)2AsO.OH, obtained by the oxidation of
      cacodyl, and having the properties of an exceedingly
      stable acid; -- also called alkargen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cacodylic
    adj 1: of or relating to cacodyl