slug (n.1)

"shell-less land snail," 1704, originally "lazy person" (early 15c.); related to sluggard.

slug (n.2)

"lead bit," 1620s, perhaps a special use of slug (n.1), perhaps on some supposed resemblance. Meaning "token or counterfeit coin" first recorded 1881; meaning "strong drink" first recorded 1756, perhaps from slang fire a slug "take a drink," though it also may be related to Irish slog "swallow." Journalism sense is from 1925, originally a short guideline for copy editors at the head of a story.

slug (n.3)

"a hard blow," 1830, dialectal, of uncertain origin; perhaps related to slaughter or perhaps a secondary form of slay.

slug (v.)

"deliver a hard blow with the fist," 1862, from slug (n.3). Related: Slugged; slugging. Slugging-match is from 1878.

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Definitions of slug from WordNet
slug (n.)
a projectile that is fired from a gun;
Synonyms: bullet
slug (n.)
a unit of mass equal to the mass that accelerates at 1 foot/sec/sec when acted upon by a force of 1 pound; approximately 14.5939 kilograms;
slug (n.)
a counterfeit coin;
slug (n.)
an idle slothful person;
Synonyms: sluggard
slug (n.)
an amount of an alcoholic drink (usually liquor) that is poured or gulped;
he took a slug of hard liquor
slug (n.)
a strip of type metal used for spacing;
Synonyms: type slug
slug (n.)
any of various terrestrial gastropods having an elongated slimy body and no external shell;
slug (n.)
(boxing) a blow with the fist;
Synonyms: punch / clout / poke / lick / biff
slug (v.)
strike heavily, especially with the fist or a bat;
He slugged me so hard that I passed out
Synonyms: slog / swig
slug (v.)
be idle; exist in a changeless situation;
He slugged in bed all morning
Synonyms: idle / laze / stagnate