Source : Webster's Revised Unabridged Dictionary (1913)

Swingletree \Swin"gle*tree`\, n. [So named in allusion to its
   swinging. See {Swingle}, v. i., and cf. {Swingtree}.]
   A whiffletree, or whippletree. See {Singletree}.

Source : WordNet®

swingletree
     n : 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}]