That’s the dandy; i.e. the ton, the clever thing; an expression of similar import to “That’s the barber.” See Barber.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Dancers * Dandy grey RussetNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes