A farthing. A merry grig; a fellow as merry as a grig: an allusion to the apparent liveliness of a grig, or young eel.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Grey Parson * GrimNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes