A piece of lead of any shape, to be fired from a blunderbuss. To fire a slug; to drink a dram.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Slubber De Gullion * Slug-a-bedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes