Definition of tread-mill:
(v) :
(intransitive) To step or walk (on or across something); to trample.
(v) :
(transitive) To step or walk upon.
(v) :
(figuratively, with certain adverbs of manner) To proceed, to behave (in a certain manner).
(v) :
To beat or press with the feet.
(v) :
To work a lever, treadle, etc., with the foot or the feet.
(v) :
To go through or accomplish by walking, dancing, etc.