The Collaborative International Dictionary of English v.0.48:

Copperhead \Cop"per*head`\, n. [From its color.]
   1. (Zool.) A poisonous American serpent (Ancistrodon
      conotortrix), closely allied to the rattlesnake, but
      without rattles; -- called also copper-belly, and red
      viper.
      [1913 Webster]

   2. A nickname applied to a person in the Northern States who
      sympathized with the South during the Civil War. [U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

copperhead
    n 1: common coppery brown pit viper of upland eastern United
         States [syn: copperhead, Agkistrodon contortrix]
    2: venomous but sluggish reddish-brown snake of Australia [syn:
       copperhead, Denisonia superba]