Green’s Dictionary of Slang

(UK/US Und.) one who works as the intermediary between a thief and a receiver of stolen goods.

1859		Matsell Vocabulum 46: janasmug A go-between; one who goes between the thief and the fence.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant I 494/1: Janasmug (thieves), a go-between; one who was intermediary between a thief and the ‘fence,’ or receiver of stolen goods.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		C. Crowe Aus. Sl. Dict. : Janasmug, a go between; one who goes between the thief’s‘fence’ .
1941		Baker Popular Dict. Aus. Sl. 38: Janusmug: A go-between.