Green’s Dictionary of Slang

ark-ruff n.

also ark-pirate, -ruffian, ack-pirate, -ruff, -ruffian
[var. on ark-man n.]

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

[UK]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.
[UK]Bailey Universal Etym. Eng. Dict.
[UK]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.
[UK]H.T. Potter New Dict. Cant (1795) n.p.: ark pirates, fresh-water thieves who steal on navigable rivers.
[UK]G. Andrewes Dict. Sl. and Cant.
[UK]Lex. Balatronicum.
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]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.
[UK]Flash Dict. in Sinks of London Laid Open.
[UK]Duncombe New and Improved Flash Dict. n.p.: Ark Pirates fresh water thieves who plunder barges and other craft on rivers.
[US]Matsell Vocabulum 7: ackruffs River-thieves; river-pirates.
[UK]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.
[US]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.
[US]Trumble Sl. Dict. (1890).
[US]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’.
[UK]Farmer Americanisms 5: Ackruffs. — An old slang expression for river thieves.