Green’s Dictionary of Slang

passing off a worthless article as valuable; also attrib.

1810		New & Complete Amer. Encyc. 50/1: Duffing out the old pursuits with these new ornaments, gives them an additional dignity and lustre.
1826		London Mag. June 173: There is a stratagem in old-clothes dealing called duffing. The practitioner [...] raises the scanty nap of a veteran garment, gives it a gloss [...] and passes it off as new .
1826		Pierce Egan’s Life in London 2 Apr. 494/1: A summons was granted to a Jew, named Moses, against a Pawnbroker [...] for having practised a ‘duffing’ trick upon him.
1838		R.S. Surtees Jorrocks Jaunts (1874) 104: The chaps about him [...] were in the duffing line—sold brimstoned sparrows for canary-birds, Norwich shawls for real Cashmere, and dried cabbage-leaves for cigars.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 380/2: But ‘duffing,’ and all that is going down fast, and I wish it was gone altogether. [Ibid.] II 19/2: What I have called the ‘dishonest trade’ is known among the street-folk as ‘music-duffing’. [Ibid.] II 70/1: In parrot-selling there is a good deal of ‘duffing’.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 9: Ring duffing:Selling a ring on the pretext that it has been stolen.