Green’s Dictionary of Slang

chained up, fettered; thus unslanged, freed of fetters, let out of prison.

a.1790		H.T. Potter New Dict. Cant (1795) n.p.: slanged ironed on one leg.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum n.p.: Slang. A fetter. Double slanged; double ironed. Now double slanged into the cells for a crop he is knocked down; he is double ironed in the condemned cells, and ordered to be hanged.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 265: slang’d: fettered.
1835		Metropolitan Mag. XIV Sept. 333: I waited until he was unslang’d and come up to London.
1848		Flash Dict. in Sinks of London Laid Open 124: Slanged, ironed on one leg.
1859		Matsell Vocabulum.