The Collaborative International Dictionary of English v.0.48:

Cacodyl \Cac"o*dyl\, n. [Gr. ??????? ill-smelling (kako`s bad +
   ????? to smell) + -yl.] (Chem.)
   Alkarsin; a colorless, poisonous, arsenical liquid,
   As2(CH3)4, spontaneously inflammable and possessing an
   intensely disagreeable odor. It is the type of a series of
   compounds analogous to the nitrogen compounds called
   hydrazines. [Written also cacodyle, and kakodyl.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

cacodyl
    n 1: a poisonous oily liquid with a garlicky odor composed of 2
         cacodyl groups; undergoes spontaneous combustion in dry air
         [syn: cacodyl, tetramethyldiarsine]
    2: the univalent group derived from arsine [syn: cacodyl,
       cacodyl group, cacodyl radical, arsenic group]