IPA: /ˈtrɛd.lər/
KK: /ˈtrɛd.lər/
A person who operates a treadle, which is a foot-operated lever used in various machines, especially in sewing or weaving.
The treadler skillfully operated the sewing machine with her foot.
The word 'treadler' originates from the Old English 'tredan' (meaning to tread or step) combined with the agent suffix '-er' (meaning a person who). A treadler is a person who treads or steps, often referring to someone who operates a treadle or similar mechanism.
Think of someone who 'steps' ('tredan') to operate a machine — that's why a treadler is a person who treads.