E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Brother Jonathan. In the revolutionary war, Washington, being in great want of supplies for the army, and having unbounded confidence in his friend, Jonathan Trumbull, governor of Connecticut, said, We must consult brother Jonathan. Brother Jonathan was consulted on all occasions by the American liberator, and the phrase becoming popular was accepted as the national name of the Americans as a people.