Saddle. (Latin, sella; French, selle.)
“The horse ... stopped his course by degrees, and went with his rider ... into a pond to drink; and there sat his lordship upon the sally.” —Lives of the Norths.
Vaulting ambition ... o'erleaps its sell, And falls o' the other ...
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894