Cheats who pretend they were farmers in the isle of Sky, or some other remote place, and were ruined by a flood, hurricane, or some such public calamity: or else called sky farmers from their farms being IN NUBIBUS, ‘in the clouds.’
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Sky Blue * Sky ParlourNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes