A glass or small measure of any spirituous liquors, which, being originally sold by apothecaries, were estimated by drams, ounces, &c. Dog’s dram; to spit in his mouth, and clap his back.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Drain * Dram-a-tickNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes