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