Copperhead was a term applied to northerners who favored the Confederate cause during the Civil War. The name came from the poisonous snake, which had a reputation for striking and injecting its venom with no warning.
The Copperheads were opposed to the abolition of slavery, sought to undermine the Lincoln administration, and openly agitated against the Civil War, urging that peace be made with the south. The best-known Copperhead was an Ohio politician, C.L. Vallandigham, who at one point was arrested by the Union Army and sent into exile to the Confederacy.