Real Dictionary

slug
countable noun: A slug is a small slow-moving creature with a long soft body and no legs, like a snail without a shell.
countable noun: If you take a slug of an alcoholic drink, you take a large mouthful of it.

slug in American English 1
any of a large number of small, gastropod mollusks, esp. the ones resembling a land snail, but having only a rudimentary internal shell buried in the mantle
noun: any of various snaillike terrestrial gastropods having no shell or only a rudimentary one, feeding on plants and a pest of leafy garden crops
transitive verb: to make (corrections) by replacing entire lines of type, esp. as set by a Linotype

slug in American English 2
a small piece or lump of metal; specif., a bullet
to insert (a slug) between lines
intransitive verb: to hit or be capable of hitting hard
transitive verb: to strike heavily; hit hard, esp. with the fist

slug in American English 3
a single drink, esp. of straight alcoholic liquor

slug in American English 4
a hard blow or hit
to hit hard, esp. with the fist or a bat

slug in British English 1
noun: any of various terrestrial gastropod molluscs of the genera Limax, Arion, etc, in which the body is elongated and the shell is absent or very much reduced

slug in British English 2
noun: an fps unit of mass; the mass that will acquire an acceleration of 1 foot per second per second when acted upon by a force of 1 pound. 1 slug is approximately equal to 32.17 pounds

slug in British English 3
verb: to hit very hard and solidly, as in boxing