Meaning

noun an exercise device consisting of an endless belt on which a person can walk or jog without changing place

noun a mill that is powered by men or animals walking on a circular belt or climbing steps

noun a job involving drudgery and confinement

originFrom tread + mill. Its figurative senses refer to the idea that running on a treadmill requires continued effort and motion to remain in the same place.

Etymology from Wiktionary, CC BY-SA 3.0