The Collaborative International Dictionary of English v.0.48:

Arsenic \Ar*sen"ic\, a. (Chem.)
   Pertaining to, or derived from, arsenic; -- said of those
   compounds of arsenic in which this element has its highest
   equivalence; as, arsenic acid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arsenic \Ar"se*nic\ ([aum]r"s[-e]*n[i^]k; 277), n. [L.
   arsenicum, Gr. 'arseniko`n, 'arreniko`n, yellow orpiment,
   perh. fr. 'arseniko`s or better Attic 'arreniko`s masculine,
   'a`rrhn male, on account of its strength, or fr. Per.
   zern[imac]kh: cf. F. arsenic.]
   1. (Chem.) One of the elements, a solid substance resembling
      a metal in its physical properties, but in its chemical
      relations ranking with the nonmetals. It is of a
      steel-gray color and brilliant luster, though usually dull
      from tarnish. It is very brittle, and sublimes at 356[deg]
      Fahrenheit. It is sometimes found native, but usually
      combined with silver, cobalt, nickel, iron, antimony, or
      sulphur. Orpiment and realgar are two of its sulphur
      compounds, the first of which is the true arsenicum of the
      ancients. The element and its compounds are active
      poisons. Specific gravity from 5.7 to 5.9. Atomic weight
      75. Symbol As.
      [1913 Webster]

   2. (Com.) Arsenious oxide or arsenious anhydride; -- called
      also arsenious acid, white arsenic, and ratsbane.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arsenic
    n 1: a white powdered poisonous trioxide of arsenic; used in
         manufacturing glass and as a pesticide (rat poison) and
         weed killer [syn: arsenic, arsenic trioxide, arsenous
         anhydride, arsenous oxide, white arsenic, ratsbane]
    2: a very poisonous metallic element that has three allotropic
       forms; arsenic and arsenic compounds are used as herbicides
       and insecticides and various alloys; found in arsenopyrite
       and orpiment and realgar [syn: arsenic, As, atomic
       number 33]

The Devil's Dictionary (1881-1906):

ARSENIC, n.  A kind of cosmetic greatly affected by the ladies, whom
it greatly affects in turn.

    "Eat arsenic?  Yes, all you get,"
        Consenting, he did speak up;
    "'Tis better you should eat it, pet,
        Than put it in my teacup."
                                                             Joel Huck