treadmill n.

Old English tredan "to tread, step on, trample; traverse, pass over" (class V strong verb; past tense træd, past participle treden), from Proto-Germanic *tred- (source also of Old Saxon tredan, Old Frisian treda, Middle Dutch treden, Old High German tretan, German treten, Gothic trudan, Old Norse troða), from PIE *der- (1) "assumed base of roots meaning 'to run, walk, step'" [Watkins]. Related: Trod; treading. To tread water in swimming, "to move the feet and hands regularly up and down while keeping the body in an erect position in order to keep the head above the water" is attested by 1764.

Middle English mille, "building fitted to grind grain," Old English mylen "a mill" (10c.), an early Germanic borrowing from Late Latin molina, molinum "mill" (source of French moulin, Spanish molino), originally fem. and neuter of molinus "pertaining to a mill," from Latin mola "mill, millstone," related to molere "to grind," from PIE root *mele- "to crush, grind." The -n- gradually was lost in English but survives in the surname Milner. Also from Late Latin molina, directly or indirectly, are German Mühle, Old Saxon mulin, Old Norse mylna, Danish mølle, Old Church Slavonic mulinu.

The meaning "mechanical device for grinding grain for food" is from 1550s. The broader sense of "machine for grinding or pulverizing any solid substance" is attested from 1670s. Other types of manufacturing machines driven by wind or water, that transform raw material by a process other than grinding began to be called mills by early 15c. Sense of "large building fitted with machinery for manufacturing" is from c. 1500. In old slang also "a typewriter" (1913); "a boxing match or other pugilistic bout" (1819).