The story
Alteration of swingletree as if from single + tree, on the analogy of doubletree.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A bar behind draft animals and in front of a load, such as a wagon, that balances the load. Generally the animals are attached at the ends and the wagon or other load to a pivot in the middle of the singletree. Full definition →
Alteration of swingletree as if from single + tree, on the analogy of doubletree.
Etymology from Wiktionary, CC BY-SA 4.0.