copperhead (n.) Look up copperhead at
Trigonocephalus contortrix, 1775, American English, so called for color markings between its eyes; see copper + head (n.). Poisonous "sneak snakes" (because they bite without warning), the name is said to have been first used in reference to Northerners suspected of Southern sympathies in Greeley's New York "Tribune," July 20, 1861. Charles H. Coleman, "The Use of the Term 'Copperhead' During the Civil War" ["Mississippi Valley Historical Review" 25 (1938), p.263] traces it to an anonymous letter against Ohio anti-war Democrats in the Cincinnati "Commercial" newspaper in the summer of 1861. It seems not to have been in widespread use until summer 1862.