He will not find out a way to set the Thames on fire; he will not make any wonderful discoveries, he is no conjuror.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Tetbury Portion * Thatch-gallowsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes