The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
swingletree 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]