Green’s Dictionary of Slang

cubit, the n.

[William Cubitt (1785–1861) who invented the treadmill (albeit for grinding corn), which, from 1818, was introduced into British prisons as a form of punishment]

the treadmill, as employed in prisons; thus punishment by the cubit, a spell on the treadmill.

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[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.