This is a defunct thieves cant term for the gallows, first recorded at the end of the seventeenth century.
I will shew you a way to empty the pocket of a queer cull, without any danger of the nubbing cheat.
Tom Jones, by Henry Fielding, 1745. Cull is a fool, dupe or sucker.
It’s formed from two other obsolete words: nub, originally East Anglian dialect meaning “neck” (which is probably related to the sense of “protuberance” and to our surviving use as “the gist or point of a story”) and cheat, another item of thieves’ cant for any sort of thing or article.
In similar vein, the nubbing-cove is the hangman (using cove in the ancient sense of “man” that still survives in some places) and nubbing-ken is the court house, a name that indicated the likely fate of anyone who ended up there (ken is yet another bit of slang from the world of vagabonds, thieves and beggars meaning “house”). It often appears in works written long after its heyday as a scene-setting device:
Then to the gipsy lad, in a tone full of meaning, “The gentry mort,” she said, in thieves’ patter, “is not worth the nubbing-cheat. I’m fly, and I’ll not have it. Stow it, my lad, and don’t be a flat!” “And let you peach on us?” he answered, smiling. Lunt struck the table. “Stop your lingo!” he said.
Starvecrow Farm, by Stanley J Weyman, 1905. Gentry mort: a gentlewoman; fly: clever, sharp, wide-awake; stow: be silent; flat: a naive or foolish person; peach: to inform against someone.