An Italian paste made of flour and eggs. Also a fop: which name arose from a club, called the Maccaroni Club, instituted by some of the most dressy travelled gentlemen about town, who led the fashions; whence a man foppishly dressed, was supposed a member of that club, and by contraction styled a Maccaroni.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Mace CoveNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes