treadle (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.

instrumental word-forming element, expressing "appliance, tool," from Old English -ol, -ul, -el, representing PIE *-lo- (see -ule). In modern English usually -le except after -n-. As in treadle, ladle, thimble, handle, spindle, girdle; also compare dialectal thrashle "flail, implement for thrashing," from Old English ðerscel, Middle English scrapel "instrument for scraping" (mid-14c.), etc.