The Collaborative International Dictionary of English v.0.48:

Toxic \Tox"ic\, Toxical \Tox"ic*al\, a. [L. toxicum poison,
   originally, a poison in which arrows were dipped, Gr.
   toxiko`n (sc. ?) poison for smearing arrows with, fr.
   toxiko`s of or for the bow, from to`xon bow, arrow. Cf.
   Intoxicate.]
   Of or pertaining to poison; poisonous; as, toxic medicines.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toxic
    adj 1: of or relating to or caused by a toxin or poison;
           "suffering from exposure to toxic substances" [ant:
           atoxic, nontoxic]