Scottish word which comes from ScottishGaelic - the literal meaning of 'dram' in English is simply 'drink', so it doesn't really have to be just whisky/spirits though it's normally used for these things.
Ex.1 'Gies a dram' (Please give me a drink, good sir)
Ex.2 'Gies a dram or I'll f**kin' chib yer heid' (I would very much appreciate a drink as soon as is convenient for you)
dram 1 (drm)
n.
1. Abbr. dr.
a. A unit of weight in the U.S. Customary System equal to 1/16 of an ounce or 27.34 grains (1.77 grams).
b. A unit of apothecary weight equal to 1/8 of an ounce or 60 grains (3.89 grams). See Table at measurement.
2.
a. A small draft: took a dram of brandy.
b. A small amount; a bit: not a dram of compassion.
Middle English dragme, a drachma, a unit of weight, from Old French, from Late Latin dragma, from Latin drachma; see drachma.
3. A small amount of anything as in a wee dram (wee means very small)
I believe I'll have a wee dram of Scotch Whisky before I turn in tonight.