The Collaborative International Dictionary of English v.0.48:

Thill \Thill\, n. [OE. thille, AS. ?ille a board, plank, beam,
   thill; akin to ?el a plank, D. deel a plank, floor, G. diele,
   OHG. dili, dilla, Icel. ?ilja a plank, planking, a thwart,
   ?ili a wainscot, plank; cf. Skr. tala a level surface.
   [root]236. Cf. Fill a thill, Deal a plank.]
   1. One of the two long pieces of wood, extending before a
      vehicle, between which a horse is hitched; a shaft.
      [1913 Webster]

   2. (Mining) The floor of a coal mine. --Raymond.
      [1913 Webster]

   Thill coupling, a device for connecting the thill of a
      vehicle to the axle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thill
    n 1: one of two shafts extending from the body of a cart or
         carriage on either side of the animal that pulls it