Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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