The Collaborative International Dictionary of English v.0.48:

Treadle \Trea"dle\, n. [OE. tredyl a step, AS. tredel. See
   Tread.]
   1. The part of a foot lathe, or other machine, which is
      pressed or moved by the foot.
      [1913 Webster]

   2. (Biol.) The chalaza of a bird's egg; the tread.
      [1913 Webster]

WordNet (r) 3.0 (2006):

treadle
    n 1: a lever that is operated with the foot [syn: pedal,
         treadle, foot pedal, foot lever]
    v 1: tread over; "the brick maker treadles over clay to pick out
         the stones"
    2: operate (machinery) by a treadle

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "treadle":
   advance, bar, beam, boom, bowl, bunt, butt, cant hook, claw bar,
   crank, crow, crowbar, drive, forward, handspike, impel, iron crow,
   jimmy, lever, limb, marlinespike, move, outrigger, peavey, pedal,
   pinch bar, pole, prize, propel, pry, push, ripping bar, roll, row,
   shove, shunt, spar, sweep, sweep along, thrust, troll, trundle,
   wrecking bar