Green’s Dictionary of Slang

(a) (also knockabout hand) a riverside thief.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Lumpers. [...] also thieves who lurk about wharfs, & pilfer goods from ships, lighters, &c.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.:
1797		P. Colquhoun Police of the Metropolis 63: The Lumpers [...] have the largest share of the plunder on the River [...] they are generally furnished with two pairs of trowsers, and with frocks [...] with large pockets, for the greater convenience of concealing and conveying plunder.
1811		Lex. Balatronicum [as cit. 1788].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1788].
1839		W.A. Miles Poverty, Mendicity and Crime; Report 58: Robberies on mercantile goods are annually committed to an enormous extent, by men called ‘lumpers’.
1857		Dickens ‘Down with the Tide’ in Reprinted Pieces (1899) 199: The Lumpers dispose of their booty easily to marine store dealers [...] Lumpers also smuggle goods ashore for the crews of vessels.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1873		Sl. Dict.
1895		C. Crowe Aus. Sl. Dict. 47: Lumper, a thief who haunts wharves and robs vessels.
1949		Monteleone Criminal Sl. (rev. edn).

(b) the ‘lowest order and more contemptible species’ of thief who lurk and grab whatever they can, regardless of value.

1781		G. Parker View of Society II 78: Lumpers, the lowest order and most contemptible species of thieving, for even in thieving there are gradations; and they look down from a superior upon those in an inferior rank with more contempt than a Peer would on a Porter. They have been expelled from the society of their brethren for being unable to scamp, prig, or dive, and they then commence Lumpers, which is skulking about ships, lighters, &c., hanging about quays, wharfs, &c. stealing old iron, fruit, sugar, or whatever comes to hand.
1789		G. Parker Life’s Painter 143: Shew me the person that when the neddies are posted, or pinched hard, hat won’t snitch, from high treason, Lutterlough, down to a gallows lumper.
1924		Lawrence & Skinner Boy in Bush 261: Of all the old sweeps! [...] Tell you what, you look like a lumper, absolutely nothing but a lumper.
1958		(con. 1940s) B. Behan Borstal Boy 303: A lot of bloody lumpers [...] reared to thieving and stealing and living off prostitutes.

(c) a seller of goods under false pretences, the old made to look new, the weak strong etc.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 373/1: A ‘lumper’ would sell linens, cottons, or silks, which might be really the commodities represented; but which, [...] were made to appear new when they were old.
1873		Sl. Dict.

(d) (US prison) a convict who is an orderly.

1953

T. Runyon In For Life 67: The gallery lumper, a kind of convict orderly who is an institution within an institution, as natural to prison as a guard tower.

(a) a contractor or a worker who loads and unloads heavy cargo, orig. ship’s cargo [cited as slang in Grose 1785].

1788		Proceedings on the King's Commissions of the Peace, Oyer and Terminer 83/1: The lumpers have no right to break any package on board the ship.
1800		P. Colquhoun Commerce and Police of the River Thames 105: Nor were those Lumpers [...] searched but allowed to go on shore, three times a day, generally laden with Plunder.
1847		Liverpool Mercury 5 Nov. 8/6: Matthew Battle, usually employed as a lumper about the docks, after swindling several persons [...] absconded.
1857		Dickens ‘Down with the Tide’ in Reprinted Pieces (1899) 199: Then there were the lumpers, or labourers employed to unload vessels. They wore loose canvas jackets with a broad hem in the bottom, turned inside, so as to form a large circular pocket in which they could conceal, like clowns in pantomimes, packages of surprising sizes.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 93/2: Then the ‘lumpers,’ or those engaged in discharging the timber ships.
1873		Falkirk Herald 10 July 7/2: Mr Cornelius Gibb, lumper of Clackmannan, the gentleman of African descent.
1877		G. Bunster in Wkly Times (Melbourne) 8 Sept. 9/2: He had worked as a lumper in the docks.
1880		Bulletin (Sydney) 12 June 5/1: Commercial men [...] are to he seen on their wharves, in swallow-tails and lavender kids, watching the loading or unloading of the steamers, while the wife stands in all her bravery of dress to he stared at by the lumpers.
1889		Shields Dly Gaz. 3 Apr. 3/3: Assaulting a ‘Lumper’ [...] John Lander [...] was employed by the N.E.R. company to discharge ships.
1892		H. Lawson ‘The Press Gang’ in Roderick (1967–9) I 190: He has got to slave for Mammon same as lumpers have to do.
1900		Bulletin (Sydney) 3 Mar. 24/4: Saw two bulldogs in ‘holts’ on a Sydney wharf the other day, and half-a-dozen lumpers hanging on to each dog trying to drag them apart.
1908		Essex Newsman 8 Aug. 4/8: [headline] Harwich Lumped Drowned.
1910		Bulletin (Sydney) 21 July 11/1: ‘Lumper’: On one of the Sydney wharves last week I came across a man paying duty on several pairs of second-hand gloves.
1916		C.J. Dennis ‘Duck an’ Fowl’ in Moods of Ginger Mick 15: A vari’gated company tears the scram [...] Frum a lawyer to a common lumper-man.
1924		Lawrence & Skinner Boy in Bush 24: Whether you’re a bush-whacker or a lumper you can be a gentleman.
1953		K. Tennant Joyful Condemned 1: An unpretentious public house offered to the sailor, the coal-lumper and the dock worker a glittering array of pictures advertising beer.
1967		S. Hugill Sailortown 4: His pride in being a real sailor and not a longshore lumper.
1968		D. O’Grady A Bottle of Sandwiches 91: During a smoko break for the lumpers he brought up the subject [...] again.
1984		(ref. to 1930s) S. Macintyre Militant 67: As many as 1500 lumpers would be working round the clock.

(b) a small contractor, a middleman, an exploitative factory owner.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 330/1: The party who takes the job in the lump from the speculator usually employs a foreman, whose duty it is to give out materials, and to make the working drawings. The men to whom it is sublet only find labour, while the ‘lumper,’ or first contractor, agrees for both labour and materials.