slug (n.1) Look up slug at Dictionary.com
"shell-less land snail," 1704, originally "lazy person" (early 15c.); related to sluggard.
slug (n.2) Look up slug at Dictionary.com
"lead bit," 1620s, perhaps a special use of slug (n.1) with reference to its shape. 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."
slug (n.3) Look up slug at Dictionary.com
"a hard blow," 1830, dialectal, perhaps related to slaughter, slay, etc.
slug (v.) Look up slug at Dictionary.com
"deliver a hard blow with the fist," 1862, from slug (n.3). Related: Slugged; slugging.