Out of use, To nab the rust; to be refractory; properly applied to a restive horse, and figuratively to the human species. To ride rusty; to be sullen; called also to ride grub.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Russian Coffee-house * Rusty GutsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes