Green’s Dictionary of Slang

flukum n.

[? fluke n.2 (1), i.e. it’s a fluke that any of it gets sold; note also carnival flukum: any fom of unspecified and thus mysterious liquid]

(US tramp) cheap, nickel-plated goods sold as ‘silver-plate’.

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[US]Irwin Amer. Tramp and Und. Sl. 77: Flukum. – Nickle-plated ware as sold by pitchmen. Just why it was given this name is hard to say, although it may be from the fact that occasionally, by some lucky chance or fluke, the pitchman is able to sell his wares quickly and depart before the ‘plating’ wears off and his customers return for their money.