Green’s Dictionary of Slang

1. worthless, false, esp. of goods sold as more valuable than they really are.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 20/1: The duffing German brass instruments might be well-toned enough, the inferiority consisting chiefly in the materials.
1867		Sportsman 17 Jan. 2/1: Notes on News [...] [A] rising generation which yells ‘Champagne Charley is my name!’ wears ‘duffing’ jewellery [and] takes its affected cynicism from the comic papers.
1868		Sportsman 2 July 2/1: Notes on News [...] [of a dishonoured cheque] [T]he cheque was dishonoured [...] Mr Wyman, the well-known printer, [...] Dr. Lester also gave a ‘duffing’ cheque.
1877		Graphic 29 Sept. 18/3: One of the oldest dodges of the street swindler is to drop a ‘duffing’ ring made in imitation of old jewellery. This trick was successfully practised [...] in Oxford Street [...] he ring-dropper is now in custody.
1884		Ally Sloper’s Half Holiday 14 June 49: [caption] ‘I’m blessed if the old humbug ain’t been and gave me a duffing ’alfpenny’.
1885		Bulletin (Sydney) 25 July 6/3: [...] we have had too many high-priced duffing pictures stuck in our gallery lately.
1896		‘’Arry on African Affairs’ in Punch 22 Feb. 90/2: The duffingest game is ’ard work on a measly small screw.
1908		Sporting Times 15 Feb. 2/5: D’jyer mean to say we’ve got ter pay to see these ’ere duffin’ kings, just as if they was a football match?

2. foolish, incompetent.

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 3/11: Oh, that’s a duffin’ dog; any dog could kill quicker than him.
1865		Leaves from Diary of Celebrated Burglar 108/2: Owing to the small rate of remuneration ‘slung’ to him by the ‘duffing’ third-rate ‘picking-up molls’ for his services in carrying the ‘cash,’ ne’er a red could be spared on new ‘toggery’.
1884		Sporting Times 15 Nov. 1/2: Either Lords Wolseley and Strathnairn are two duffing generals or in the little girl who beat them both hollow in a game of Kriegspiel, we have a Joan of Arc.
1885		Bulletin (Sydney) 18 July 18/1: Oh, my! what a pious world it is, / And how very good they all seem to be – / But what a ’duffing’ lot you’d find / If you would only raise the blind, / And see ’em on the strict Q.T.
1890		Dead Bird (Sydney) 20 Dec. 2/1: ‘Here, you duffin’ son of a sea cook, back water, and set me ashore again!’ .
1905		N. Gale ‘Uncle Bob Indignant’ in More Cricket Songs 21: By George, there goes the supper-bell! / And yet your duffing Uncle Bob / Has never told you what befell.