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slug 1

1. A round bullet larger than buckshot.
2. Informal
a. A shot of liquor.
b. An amount of liquid, especially liquor, that is swallowed in one gulp; a swig.
3. A small metal disk for use in a vending or gambling machine, especially one used illegally.
4. A lump of metal or glass prepared for further processing.
5. Printing
a. A strip of type metal, less than type-high and thicker than a lead, used for spacing.
b. A line of cast type in a single strip of metal.
c. A compositor's type line of identifying marks or instructions, inserted temporarily in copy.
6. Physics The British unit of mass that accelerates at the rate of one foot per second per second when acted on by a force of one pound on the surface of the Earth.
tr.v. slugged, slug·ging, slugs
1. Printing To add slugs to.
2. Informal To drink rapidly or in large gulps: slugged down a can of pop.

[Perhaps from slug (from its shape).]

slug 2

1. Any of various terrestrial gastropod mollusks having a slow-moving slimy elongated body with no shell or with a flat rudimentary shell on or under the skin, usually found in moist habitats.
2. A sea slug.
3. The smooth soft larva of certain insects, such as the sawfly.
4. A slimy mass of aggregated amoeboid cells that develops into the spore-bearing fruiting body of a cellular slime mold.
5. Informal A sluggard.

[Middle English slugge, sluggard, probably of Scandinavian origin.]

slug 3

tr.v. slugged, slug·ging, slugs
To strike heavily, especially with the fist or a bat.
A hard heavy blow, as with the fist or a baseball bat.

[Possibly from slug.]

slug 4

intr.v. slugged, slug·ging, slugs
To wait for or obtain a ride to work by standing at a roadside hoping to be picked up by a driver who needs another passenger to use the HOV lanes of a highway.
A commuter who slugs.

[Probably from slug.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in classic literature ?
So let us cheer up, and have a run to the other end of the orchard; I believe the wind has blown down some apples, and we might just as well eat them as the slugs."
The Moors have some small silver coins and also some silver slugs worth a dollar each.
My grandfather had had it made, and, like other buildings, it enjoyed a period of prosperity before being left to the ravages of slugs and children, when he came down every afternoon in summer and drank his coffee there and read his Kreuzzeitung and dozed, while the rest of us went about on tiptoe, and only the birds dared sing.
However, I said no more to the boy, but bade him lie still, and I took our biggest gun, which was almost musket-bore, and loaded it with a good charge of powder, and with two slugs, and laid it down; then I loaded another gun with two bullets; and the third (for we had three pieces) I loaded with five smaller bullets.
On the one hand, he surprised himself by his discoveries in natural history, finding that his piece of garden-ground contained wonderful caterpillars, slugs, and insects, which, so far as he had heard, had never before attracted human observation; and he noticed remarkable coincidences between these zoological phenomena and the great events of that time,--as, for example, that before the burning of York Minster there had been mysterious serpentine marks on the leaves of the rose-trees, together with an unusual prevalence of slugs, which he had been puzzled to know the meaning of, until it flashed upon him with this melancholy conflagration.
There were bundles of labels, cupboards, and drawers with compartments, and wire guards for the cupboards, to allow free access to the air whilst keeping out slugs, mice, dormice, and rats, all of them very curious fanciers of tulips at two thousand francs a bulb.
Loaded with two slugs and a round bullet, its effect was that of a sawed-off shotgun.
And his union was compelled to bully and slug the employers' unions in order to hold up wages or hold down hours.
The sixth, that it doth dull and damp all industries, improvements, and new inventions, wherein money would be stirring, if it were not for this slug. The last, that it is the canker and ruin of many men's estates; which, in process of time, breeds a public poverty.
This slug, when disturbed, emits a very fine purplish-red fluid, which stains the water for the space of a foot around.
It is the father and the mother of the commonplace, and it is the commonplace, like a loathsome slug, that beslimes and destroys love.
In fact, of the twenty rose-trees which formed the parterre, not one bore the mark of the slug, nor were there evidences anywhere of the clustering aphis which is so destructive to plants growing in a damp soil.