Green’s Dictionary of Slang

coonskin n.

[? as used in the fur trade, when furs were the barterers’ equivalent of cash]

(US) a $1 bill; also attrib.

[US]W.C. Hall ‘Mike Hooter’s Bar Story’ in Spirit of the Times 26 Jan. (N.Y.) 581: That feller Bonnel sold me a pint of red-eye whiskey [...] for a coonskin, an’ then guv me a brass picayune for change.
[US]‘Edmund Kirke’ Down in Tennessee 187: He reckoned ‘thet such ruin [...] fur a coonskin a gallon, was purty tollable sort o’ ruin’.
[US]Wash. Post (DC) 14 Aug. 4/3: He was going to come pretty close to toting home the coonskins.
[US]Botkin A Treasury of Amer. Folklore 322: Something, too, of the conservative’s distrust of the pioneer and ‘coonskin’ democracy enters into the treatment of backwoods hospitality.

In exclamations

by the eternal coonskins!

(US) a general oath.

[US]St Louis Republican (MO) 5 Apr. 61/3: By the eternal ’coonskins!
Mitchell Capital (Dakota, SD) 16 June 12: ‘By the eternal ’coonskins! I can gouge the eye out of any man’.