Green’s Dictionary of Slang

(UK Und.) a gold or silver chain; thus nip the slag, to cut a watch chain.

1857		‘Ducange Anglicus’ Vulgar Tongue 19: Slag, n. Chain, a gold or silver one.
1902		Wash. Times (DC) 14 Sept. 10/4: Slag— A watch chain.
1929		Detective Fiction Weekly 2 Mar. 694/2: One ‘nips the slag’ when one cuts the watch chain, a practice practically defunct now.
1949		Monteleone Criminal Sl. (rev. edn).