Green’s Dictionary of Slang

(UK Und.) one who searches the streets for nails, bits of iron etc.

1821		Flash Dict. n.p.: Null-gropers people who sweep the streets, in search of old irons, nails, &c.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict.