The Collaborative International Dictionary of English v.0.48:

Whippletree \Whip"ple*tree`\, n. [See Whip, and cf.
   Whiffletree.]
   [1913 Webster]
   1. The pivoted or swinging bar to which the traces, or tugs,
      of a harness are fastened, and by which a carriage, a
      plow, or other implement or vehicle, is drawn; a
      whiffletree; a swingletree; a singletree. See
      Singletree.
      [1913 Webster]

            [People] cut their own whippletree in the woodlot.
                                                  --Emerson.
      [1913 Webster]

   2. (Bot.) The cornel tree. --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

whippletree
    n 1: a crossbar that is attached to the traces of a draft horse
         and to the vehicle or implement that the horse is pulling
         [syn: whiffletree, whippletree, swingletree]