slug n.1

"shell-less land snail," 1704, originally "lazy person, slow, heavy fellow" (early 15c.) and related to sluggard. It was extended from persons to slow-moving animals by 1610s, and from the snails to similar soft-bodied creatures.

"heavy piece of crude metal for firing from a gun, lead bit, lead bullet not regularly formed," 1620s, perhaps a special use of slug (n.1), which at the time would have meant "lazy person or animal," perhaps on some supposed resemblance.

The meaning "token or counterfeit coin" is recorded from 1881; that of "strong drink" is recorded by 1756, perhaps from the slang phrase fire a slug "take a drink," though it also might be related to Irish slog "swallow."

In typography, "a thin blank of type metal" (1871), hence the journalism sense of "title or short guideline at the head of a news story in draft or galleys" (by 1925), short for slug-line, so called because usually it occupies one slug of type. Sometimes by error they get printed, and if the copy-editor's shorthand instruction is "dead head" or "kill widow," it can look bad.

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

"strike heavily, deliver a hard blow with the fist," 1862, from slug (n.3). Related: Slugged; slugging. Earlier it meant "be lazy, inert, or slothful; lie in bed" (early 15c., sluggen, from slug (n.1) in the "lazy person" sense). In journalism, "give a story a slug-line," by 1925, from slug (n.2). To slug it out is by 1943.