Green’s Dictionary of Slang

(UK Und.) a river thief, who robs and kills victims.

1725		New Canting Dict. n.p.: Ark Ruffians Villains, who, in Conjunction with Watermen, &c. rob and murder on the Water; by picking a Quarrel with the Passenger when they see a convenient Opportunity, and then plundering and stripping the unhappy Wretches, throw him or her over-board, &c.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Ark ruffians, (cant) rogues who in conjunction with watermen robbed and sometimes murdered on the water, by picking a quarrel with the passengers in a boat, boarding it, plundering, stripping, and throwing them overboard, &c.
a.1790		H.T. Potter New Dict. Cant (1795) n.p.: ark pirates, fresh-water thieves who steal on navigable rivers.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict. 3: Ark ruffians – rogues who in conjunction with watermen sometimes rob and murder on the water. Ack pirates – fresh water theives [sic] who steal on navigable rivers.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict. n.p.: Ark Pirates fresh water thieves who plunder barges and other craft on rivers.
1859		Matsell Vocabulum 7: ackruffs River-thieves; river-pirates.
1862		E. de la Bédollière Londres et les Anglais 312/2: ark ruffians, Elle désigne ceux qui, menant en bateau un passager, lui cherchaient une querelle d’Allemand, le volaient et finissaient quelquefois par le précipiter dans la Tamise.
1867		Nat. Police Gaz. (NY) 12 Oct. n.p.: [T]he ‘ackruffs’ have not been very busy [...] custodians of ship’s cargoes have not been troubled by visits from river thieves.
1881		Trumble Sl. Dict. (1890).
1882		Breckenridge News (Cloveport, KY) 23 Aug. 3/3: My earliest years were spent among ackneffs [sic] and when I could hardly more than walk I was an ‘angler’.
1889		Farmer Americanisms 5: Ackruffs. — An old slang expression for river thieves.