Brother Jonathan,
a phrase applied to the people of the United States, as "John Bull" is to the people of England. Washington, on assuming command of the New England Revolutionary Forces, was in great straits for arms and war material. The governor of Connecticut, Jonathan Trumbull, was a man of excellent judgment and an esteemed friend of Washington. In the emergency Washington said, "We must consult Brother Jonathan." This expression was repeated on other difficult occasions, and became a convenient name for the whole people.
Entry from Everybody's Cyclopedia, 1912.