slug 1 
(slŭg)
(slŭg)n.
1. A round bullet larger than buckshot.
2. Informal
a. A shot of liquor.
b. An amount of liquid, especially liquor, that is swallowed in one gulp; a swig.
3. A small metal disk for use in a vending or gambling machine, especially one used illegally.
4. A lump of metal or glass prepared for further processing.
5. Printing
a. A strip of type metal, less than type-high and thicker than a lead, used for spacing.
b. A line of cast type in a single strip of metal.
c. A compositor's type line of identifying marks or instructions, inserted temporarily in copy.
6. Physics The British unit of mass that accelerates at the rate of one foot per second per second when acted on by a force of one pound on the surface of the Earth.
tr.v. slugged, slug·ging, slugs
1. Printing To add slugs to.
2. Informal To drink rapidly or in large gulps: slugged down a can of pop.
[Perhaps from SLUG2 (from its shape).]