Merriam-Webster's Collegiate® Dictionary, 11th Edition
SLUG
I. noun
Etymology: Middle English slugge, of Scandinavian origin; akin to Norwegian dialect slugga to walk sluggishly
Date: 15th century
sluggard , a lump, disk, or cylinder of material (as plastic or metal): as, a. a musket ball, bullet , a piece of metal roughly shaped for subsequent processing, a $50 gold piece, a disk for insertion in a slot machine, any of numerous chiefly terrestrial pulmonate gastropods (order Stylommatophora) that are found in most parts of the world where there is a reasonable supply of moisture and are closely related to the land snails but are long and wormlike and have only a rudimentary shell often buried in the mantle or entirely absent, a smooth soft larva of a sawfly or moth that creeps like a mollusk,
5. a quantity of liquor drunk in one swallow, a detached mass of fluid (as water vapor or oil) that causes impact (as in a circulating system),
6. a strip of metal thicker than a printer's lead, a line of type cast as one piece, a usually temporary type line serving to instruct or identify, the gravitational unit of mass in the foot-pound-second system to which a pound force can impart an acceleration of one foot per second per second and which is equal to the mass of an object weighing 32 pounds,
II. transitive verb (slugged; slugging)
Date: 1912
to add a printer's slug to, to drink in gulps,
III. noun
Etymology: perhaps from slug to load with slugs
Date: 1830
a heavy blow especially with the fist,
IV. transitive verb (slugged; slugging)
Date: circa 1861
to strike heavily with or as if with the fist or a bat, fight 4b