Webster's Revised Unabridged Dictionary · 1913

Slug

Slug , noun

[Old English slugge slothful, sluggen to be slothful; compare LG. slukk low-spirited, sad, English slack, slouch, Dutch slak, slek, a snail.]

1.
A drone; a slow, lazy fellow; a sluggard. — Shakespeare
2.
A hindrance; an obstruction. [Obsolete] — Bacon
3.
(Zoology) Any one of numerous species of terrestrial pulmonate mollusks belonging to Limax and several related genera, in which the shell is either small and concealed in the mantle, or altogether wanting. They are closely allied to the land snails.
4.
(Zoology) Any smooth, soft larva of a sawfly or moth which creeps like a mollusk; as, the pear slug; rose slug.
5.
A ship that sails slowly. [Obsolete] — Halliwell
His rendezvous for his fleet, and for all slugs to come to, should be between Calais and Dover. — Pepys
6.
An irregularly shaped piece of metal, used as a missile for a gun.
7.
(Printing) A thick strip of metal less than type high, and as long as the width of a column or a page, -- used in spacing out pages and to separate display lines, etc.
Collocations (2)
Sea slug (Zoology) , (a) Any nudibranch mollusk (b) A holothurian.
Slug caterpillar , Same as Slugworm.

Slug , intransitive verb

To move slowly; to lie idle. [Obsolete]
To slug in sloth and sensual delight. — Spenser

Slug , transitive verb

To make sluggish. [Obsolete] — Milton

Slug , transitive verb

1.
To load with a slug or slugs; as, to slug a gun.
2.
To strike heavily. [Cant or Slang]

Slug , intransitive verb

To become reduced in diameter, or changed in shape, by passing from a larger to a smaller part of the bore of the barrel; -- said of a bullet when fired from a gun, pistol, or other firearm.