Word Origin & History
Trigonocephalus contortrix, 1775, Amer.Eng., so called for color markings between eyes; 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 the 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 was in widespread use from summer 1862.