Green’s Dictionary of Slang

(UK und.) a river thief.

1832		Public Ledger 12 Nov. 3/3: All kinds of plundering on the river and its banks, on board shipping, barges, &c. Light horsemen, heavy horsemen, game watermen, lightermen, scuffle hunters, copemen, &c.
1848		Era (London) 12 Nov. 8/3: [O]ur fighting contributor [...] has thus poetically classed them:- [...] copemen (l7) and dashers (18).