Green’s Dictionary of Slang

copperhead n.

[SE copperhead, a venomous snake (Agkistrodon contortrix) common in the United States]
(US)

1. [early–mid-19C] an unpleasant person; thus copperheaded adj., unpleasant.

2. [mid-19C] a Native American.

3. [mid-19C] (also cop, copper) a Northerner who backed the Confederacy; also attrib.