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”).