A peg in the heel of a shoe, taken out when it is finished. A person leaving any liquor in his glass, is frequently called upon by the toast-master to take off his heel-tap.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Heels * HellNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes