Unwilling. It went much against the grain with him, i.e. it was much against his inclination, or against his pluck.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
After-Clap * AgogNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes