Green’s Dictionary of Slang

copperhead n.

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

1. an unpleasant person; thus copperheaded adj., unpleasant.

[US]W. Irving Knickerbocker (N.Y.) VI iv: They were the men who vegetated in the mud [...] being of the race of genuine copperheads.
[US]W.G. Simms Sword and the Distaff 507: It is sich a little mean copper-headed son of a skunk that has the impudence to come here and seize the rightful property of a gentleman. [...] Come, copper head! march!

2. a Native American.

[US]Southern Literary Messenger IV 295/1: He said [...] it would be a sin to kill one, but if he was to go he should want to kill one of the damned copper-heads [DA].
[US]C.W. Webber Shot in Eye (1855) 132: He had a hatred for the ‘yaller bellies,’ and ‘copper-heads,’ as he called the Mexicans and Indians, which was refreshingly orthodox [DA].
T. Winthrop Canoe and the Saddle (1883) 145: Five foul copper heads [...] lurked among the plunder.
[US] in W.S. Walsh Literary Curiosities.

3. (also cop, copper) a Northerner who backed the Confederacy; also attrib.

[US]Lawrence Republican 11 Sept. 4/1: That faction of the Democracy who sympathises with the rebels are known [...] in Kansas as ‘jayhawkers,’ in Kentucky as ‘bushwhackers,’ and in Indiana as ‘copperheads’ [DA].
[US]C.H. Smith Bill Arp 143: Mr. Johnson ain’t got no more respect for ’em than I have. We want to trade ’em off. By hoky, we’ll give two of ’em for one copperhead, and ax nothin to boot.
[US] ‘What are they thinking of?’ Grant Songster 6: What are the ‘Coppers’ thinking of?
[US] ‘Is it for honest Loyalty?’ Grant Songster 9: Give fools their silks, and ‘Cops’ their wine.
[UK]Chambers’s Journal 13 Mar. 172/2: During the rebellion, the Peace party, being suspected of favouring the South, were nicknamed Copperheads or Cops [DA].
[UK]G.A. Sala in Living London (1883) Mar. 115: To the American Civil War the Lingua Balatronica owes the revival, if not the invention, of [...] ‘copperhead’.
[UK]G.A. Sala Things I Have Seen I 218: A group of [...] politicians, and journalists who were a little more than Democrats. They were known as ‘Copperheads.’.
[US]J.W. Carr ‘Word-List from Hampstead, N.H.’ in DN III iii 184: copperhead, n. A northerner who was opposed to the war for the preservation of the Union.
[US](con. 1908) E. Lynn Adventures of a Woman Hobo 104: ‘They call him an old Copperhead.’ [...] ‘I fought for the South.’.
[US]H. Asbury Sucker’s Progress 277: During the War the Spencer House was known as a ‘copperhead hotel,’ and the soundproof room was frequently used for meetings of Southern spies and sympathizers.
[US]S. Lewis Kingsblood Royal (2001) 237: The Deep South had again seceded from the American Constitution, and this time they were supported by more Northern Copperheads.