Green’s Dictionary of Slang

duff n.1

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

1. [late 18C+] counterfeit money, smuggled goods.

2. [late 19C] (UK Und.) constr. with the, the passing off of false jewellery.

3. [1920s] a fake.

4. [1950s+] (UK prison) tobacco that is contraband.

In phrases

at the duff

[late 19C–1910s] (Aus. Und.) dealing in counterfeit jewellery.