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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.]
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 sham chaplain came into our cells to exhort us, carrying a black bag, supposed to be full of tracts, and so often did he come that by the third day we had each stowed away at the foot of our beds a file, a brace of pistols, a pound of powder, and twenty slugs.
The Moors have some small silver coins and also some silver slugs worth a dollar each.
Even the insect world was to be defended,--that had been too long neglected, and a society for the protection of ground-worms, slugs, and mosquitos was to be incorporated without delay.
From this vegetable life in pond and in marsh will come, in time, the tiny crawling microscopic slugs which are the pioneers of that great army of life in which for the instant we five have the extraordinary duty of serving as rear guard.
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.
Seedlings, also, are destroyed in vast numbers by various enemies; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of the 357 no less than 295 were destroyed, chiefly by slugs and insects.
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.
I took the best aim I could with the first piece to have shot him in the head, but he lay so with his leg raised a little above his nose, that the slugs hit his leg about the knee and broke the bone.
She thought of the wood, and stole towards it, heedless of long grass and briers: of worms, snails, and slugs, and all the creeping things that be.
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.