An abbreviation of damnation: a vulgar term used in Kent, Sussex, and the adjacent counties, for very. Nation good; very good. A nation long way; a very long way.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Nask * Natty LadsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes