Green’s Dictionary of Slang

1. (Aus./N.Z.) a sturdy horse, standing about 15 hands high, that is sufficient for the work required.

1885		W.T. Hornaday Two Years in Jungle 284: The horses were large and rather raw-boned Australian ‘plugs’, well qualified for the work they had to do, and, as we had a fresh pair for every six miles .
1942		Dly Teleg. (Sydney) 27 July 4/6: An Aussie soldier goes to the races to bet on the gee-gees. A plug or a nag is a moke.

2. (US) an incompetent or undistinguished person; also attrib.

1848		Ladies’ Repository (N.Y.) Oct. VIII:37 316/2: Plug, [...] a nickname for a homely man.
1863		in J.D. Billings Hardtack and Coffee 72: Next came General Meade, a slow old plug, / Hurrah! Hurrah! / For he let them get away at Gettysburg.
1881		‘Bill Nye’ Bill Nye and Boomerang 50: Let us ignore the death of every plug who claims to be a James’ boy.
1899–1900		C.L. Cullen Tales of the Ex-Tanks 93: Side-aisle plugs who looked like brickyard hands.
1919		A. Baer Two & Three 23 Apr. [synd. col.] Many a manager who buys a player to plug that gap in the infield discovers that he has bought a plug, all right.
1920		S. Lewis Main Street (1921) 175: I don’t want to be a plug general practitioner all my life.
1929		A.J. Barr Let Tomorrow Come 39: Well, me an’ a plug – K.Y.; I don’t know his real handle.
1932		L.W. Merryweather ‘Argot of an Orphans’ Home’ in AS VII:6402: plug, n. A clumsy person.
1940		(con. 1917) S.J. Simonsen Soldier Bill 45: The young fellows joining the army nowadays can take the girls away from us plugs; there is something wrong with us.
1948		Redbook Mar. 48/2: You—you broken reed! You doormat! Old steady, unimaginative, dumb plug! [DA].

3. (US) a fellow, a person, a chap.

1853		Life in Boston & N.Y. (Boston, MA) 1 Oct. n.p.: I wish to show up some of the bloods of this town [...] They assembled at a favorite resort if such plugs [...] the Excelsior Saloon.
1899		J. Flynt Tramping with Tramps 278: I’m always willing to be square to a square plug [fellow].
1904		Number 1500 Life In Sing Sing 251: Plug. A fellow.
1907		C. M’Govern By Bolo and Krag 201: ‘The gentlemens Americans wish find gold,’ the grey-haired old plug had told me.
1912		D. Lowrie My Life in Prison 47: He seems t’ be a pretty good sort o’ plug.
1921		P. & T. Casey Gay-cat 302: Gee, guy, gun, mug, plug, stiff, etc.—a fellow.
1949		Monteleone Criminal Sl. (rev. edn).

4. (US) a worn-out old horse; occas. of a man (see cite 1886).

1860		O.H. Oldroyd Lincoln’s Campaign (1896) 171: There’s an old plow ‘hoss’ whose name is ‘Dug,’ [...] he’s short and thick and a regular ‘plug’ [DA].
1872		‘Mark Twain’ Roughing It 179: I know that horse [...] he is, without the shadow of a doubt, a Genuine Mexican Plug!
1881		‘Bill Nye’ Bill Nye and Boomerang 17: He jabs the Mexican spurs into the foamy flank of his noble cayuse plug.
1886		Nat. Police Gaz. (NY) 3 July 14/1: The old plug, George Washington Bradley, has again been turned out to pasture. The Rochesters had no use for h1m, as their men are all of a faster class.
1899		Kirk Munroe Forward, March 29: You see she’s a Mexican—what Mark Twain would call a ‘genuine Mexican plug’.
1906		S. Ford Shorty McCabe 138: It didn’t seem to worry Jarvis any more’n if he was drivin’ a pair of mail-wagon plugs.
1908		H. Green Maison De Shine 218: A coupla acrobats what lived here got me to put the hull roll on some old plug.
1911		G. Bronson-Howard Enemy to Society 7: So long, stick-in-the-mud. Needn’t stop your old plugs fer me.
1920		F.S. Fitzgerald This Side of Paradise in Bodley Head Scott Fitzgerald III (1960) 225: You can leave your old plug in our stable.
1931		(con. 1900) L. Riggs Green Grow the Lilacs I iii: Plug or no plug, you mighta tied him some’eres else.
1947		C.J. Lovell ‘The Background of Mark Twain’s Vocab.’ in AS XXII:2 95: plug. A worthless horse.
1955		‘Blackie’ Audett Rap Sheet 41: There was two fresh young saddle horses tied to the rail outside the barn. The old plugs was gone.
1970		E. Tidyman Shaft 1: A long purple coat that looked like a blanket on a Central park plug.

5. (US campus) a hard-working student.

1900

E.H. Babbitt ‘College Words and Phrases’ in DN II:i 49: plug, n. A hard student.

6. (Can.) an unpleasant person.

1900

E.H. Babbitt ‘College Words and Phrases’ in DN II:i 49: plug, n. A slow, disagreeable person.

7. (US) a hard-working but materially unsuccessful person.

1913

J. London Valley of the Moon (1914) 174: There was two kinds of us, the lions and the plugs. The plugs only worked, the lions only gobbled.

8. a worn-out racing greyhound.

1933

W.R. Burnett Dark Hazard (1934) 239: Tommy Mason [...] he’s got his dogs here. Most of them old plugs, no good.

9. (US) a damaged or malfunctioning object, e.g. an old car.

1982		(con. 1949) B.A. Mason ‘Detroit Skyline 1949’ in Shiloh 41: We’ve still got that old plug, but it gets us to town.
1983		S. King Christine 63: I looked at the car again, the ’58 Plymouth, sitting in here when it should have been out back in the junkyard with the rest of Darnell’s rotten plugs.

10. (N.Z. prison) a cigarette filter.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 142/2: plug n. a cigarette filter.