A meeting of citizens in America to agree upon what members
they intend to support, and to concert measures for carrying out their
political wishes. The word arose from the caulkers of Boston, who had a
dispute with the British soldiers a little before the Revolution.
Several citizens were killed, and meetings were held at the caulkers'
house or calk-house, to concert measures for redress of
grievances.
“The whole Fenian affair is merely a caucus in disguise.” —The
Times.
“This day the caucus club meets ... in the garret of Tom Dawes, the
adjutant of the Boston regiment.” —John Adams: Diary, vol. ii.
p. 164, February, 1763.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894