- a venomous snake, Agkistrodon (Ancistrodon) contortrix, of the eastern and southern U.S., having a light-brown to copper-red body marked with darker bands.
- an extremely venomous but sluggish snake, Denisonia superba, of Australia and Tasmania, having a reddish to black body, depending on the region.
- (initial capital letter) U.S. History. a Northern Democrat who opposed the Civil War, advocating peace and restoration of the Union even if slavery continued.
- (initial capital letter) Military. a finned, 155mm cannon-launched U.S. Army artillery shell that homes on the target, using the reflection of a laser beam projected by a forward observer.
- a venomous reddish-brown snake, Agkistrodon contortrix, of the eastern US: family Crotalidae (pit vipers)
- a venomous reddish-brown Australian elapid snake, Denisonia superba
- US informal a Yankee supporter of the South during the Civil War
Trigonocephalus contortrix, 1775, American English, so called for color markings between its eyes; see + (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.