Green’s Dictionary of Slang

tosher n.

[dial. toshy, muddy]

1. one who scavenges copper from ships’ bottoms, items from the Thames mud, the sewers, etc.

[UK]H. Mayhew Great World of London I 46: ‘Toshers,’ who purloin copper from ships and along shore.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor II 150/2: The sewer-hunters were formerly, and indeed are still, called by the name of ‘Toshers,’ the articles which they pick up in the course of their wanderings along the shore being known among themselves by the general term ‘tosh,’ a word more particularly applied by them to anything made of copper. [Ibid.] IV 26: ‘Toshers,’ or those who purloin copper from the ships along shore.
[UK]Hotten Sl. Dict.
[UK]Sl. Dict.
[Aus]Sydney Sl. Dict. (2 edn) 8: Toshers - ‘Wharf rats’ who steal copper and dunnage from ships.

2. a painter and decorator.

[UK]P. Theroux Family Arsenal 88: It wants to be toshed up [...] But the trouble is with toshers – they’re all villains.
[UK]R. Puxley Fresh Rabbit 14: A tosher nowadays is a painter, but in Victorian London he was a slum dweller who made a kind of living searching the sewers for valuables.