Green’s Dictionary of Slang

1. a thief who specializes in stripping the corpses of those who drowned in London’s River Thames.

1883

J. Greenwood Tag, Rag & Co. 34: ‘Do you really mean to say that there really are men who make a business of dragging the river for drowned persons, and that the boat we just saw was following the horrible business?’ I asked. ‘I don’t mean to say it’s a business,’ he replied [...] ‘occupying all a man’s time; it’s only a branch, like, of the reg’lar river rat’s means of a living.’.

2. (US Und.) a thief who steals on the river front.

1886		St Landry Democrat (Opelousas, LA) 10 July 2/5: A youngster who belongs to [...] the genus small boy known as the ‘river rat’.
1906		Cairo Bull. (IL) 16 June 3/5: The river rats are a great nuisance [...] Their chief occupation is fishing, stealing and raiding fields along the river.
1920		C.S. Montanye ‘Perfect Crime’ in Penzler Pulp Fiction (2007) 352: I’ll put you down as a river rat, a rattler grab, which means you’re a freight car crook.
1949		A.B. Hollingshead Elmtown’s Youth (1975) 81: The area below the canal [...] down by the garbage dump; where the river rats live; behind the tannery; the bush apes’ home; squatters’ paradise; where you’ll find the God-damned yellow hammers.
1949		Monteleone Criminal Sl. (rev. edn).

3. (US) a dock labourer.

1932		J. Peterkin Bright Skin 192: I can’ stand to think about you stayin amongst all dem river-rats an’ drunk women.
1935		J. Conroy World to Win 334: He’d been a river roustabout – a river rat, when he could get any work. And that sure puts the muscle on a man.