In or to a merry pin; almost drunk: an allusion to a sort of tankard, formerly used in the north, having silver pegs or pins set at equal distances from the top to the bottom: by the rules of good fellowship, every person drinking out of one of these tankards, was to swallow the quantity contained between two pins; if he drank more or less, he was to continue drinking till he ended at a pin: by this means persons unaccustomed to measure their draughts were obliged to drink the whole tankard. Hence when a person was a little elevated with liquor, he was said to have drunk to a merry pin.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Pimple * Pin BasketNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes