invented (and named) 1822; originally an instrument of prison discipline; from tread (v.) + mill (n.1). Treadwheel as a similar method of driving machinery is from 1570s.
As a corrective punishment, the discipline of the stepping mill has had a most salutary effect upon the prisoners, and is not likely to be easily forgotten, while it is an occupation which by no means interferes with, nor is calculated to lessen the value of, those branches of prison regulation which provide for the moral and religious improvement of the criminal. ["Description of the Tread Mill Invented by Mr. William Cubitt of Ipswich for the Employment of Prisoners," London, 1822]
By later generations regarded as a path to physical fitness.