A horse and buggy word.There were several ways of hitching a horse to a vehicle. The one I remember was the singletree.
singletree
n.
5altered (as if < SINGLE) < swingletree < ME swingle, rod, whip + tre, TREE6 a wooden bar swung at the center from a hitch on a plow, wagon, etc. and hooked at either end to the traces of a horse‘s harness
It was also called a whippletree, and even more commonly a whiffletree.
"The old gray mare, she pffft on the whiffletree, pfft on the whiffletree, many long years ago...."

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