Green’s Dictionary of Slang

duff n.1

[? Yorks. dial. duff, to avoid, to dodge]

1. counterfeit money, smuggled goods.

[UK]G. Parker View of Society II 158: Sham Leggers. The Duff. Whispering Dudders. These are divided into several classes: some travel on horseback, and some on foot; some with carts and waggons, &c. They frequent the out-skirts of cities, large towns, markets, villages, and fairs. The goods they have for sale are damaged, which they get from on board ships or out of large manufactories; but tho’ damaged, they are generally of the newest fashions and neatest patterns. [...] They endeavour to make you believe that the goods they sell are smuggled, tho’ they were really manufactured in Spitalfields.
[UK]R.L. Stevenson Treasure Island 92: Here’s to ourselves, and hold your luff, plenty of prizes and plenty of duff.
[US]H.L. Williams Love and Lockjaw 7: Lucky my money won’t do him any good. It is duff that I carry for a hold-up [DA].
[US]Bluefield Daily Tel. (WV) 11 Mar. 4/2: In addition [...] the following [names for money] are given: Soap, Long Green, Stuff, Duff, [...] Wampum.

2. (UK Und.) constr. with the, the passing off of false jewellery.

implied in at the duff

3. a fake.

[UK]T. Norman Penny Showman 6: Be that as it may, it was a good duff (fake).

4. (UK prison) tobacco that is contraband.

[UK]J. Phelan Tramp at Anchor 176: He snooks a balmy Judas, and sees him coppa duff.
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. [Internet] Duff. Tobacco (Vic).

In phrases

at the duff

(Aus. Und.) dealing in counterfeit jewellery.

[UK] ‘Autobiog. of a Thief’ in Macmillan’s Mag. (London) XL 502: The following people used to go in there [i.e. an underworld public house] — toy-getters (watch-stealers), [...] men at the duff (passing false jewellery), welshers (turf-swindlers), and skittle sharps.
[UK]G.R. Sims in Cassell’s Sat. Journal 31 Mar. 7: The man at the duff palms off false jewellery as real [F&H].
[Aus]Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 69: Men at the duff – passers of false jewellry.