From the second edition (1989):
(ˈkɔːkəs) [Arose in New England: origin obscure.
Alleged to have been used in Boston U.S. before 1724; quotations go back to 1763. Already in 1774 Gordon (Hist. Amer. Rev.) could obtain no ‘satisfactory account of the origin of the name’. Mr. Pickering, in 1816, as a mere guess, thought it ‘not improbable that caucus might be a corruption of caulkers', the word “meetings” being understood’. For this, and the more detailed statement quoted in Webster, there is absolutely no evidence beyond the similarity of sound; and the word was actually in use before the date (1770) of the event mentioned in Webster. Dr. J. H. Trumbull (Proc. Amer. Philol. Assoc. 1872) has suggested possible derivation from an Algonkin word cauʹ-cau-asʹu, which occurs in Capt. Smith's Virginia 23, as Caw-cawaassough ‘one who advises, urges, encourages’, from a vb. meaning primarily ‘to talk to’, hence ‘to give counsel, advise, encourage’, and ‘to urge, promote, incite to action’. For such a derivation there is claimed the general suitability of the form and sense, and it is stated that Indian names were commonly taken by clubs and secret associations in New England; but there appears to be no direct evidence.]

1. In U.S. A private meeting of the leaders or representatives of a political party, previous to an election or to a general meeting of the party, to select candidates for office, or to concert other measures for the furthering of party interests; opprobriously, a meeting of ‘wire-pullers’.

1763 J. Adams Diary Feb. Wks. II. 144 (Bartlett) This day learned that the caucus club meets, at certain times, in the garret of Tom Dawes. 1788 W. Gordon Hist. Amer. Rev. I. 240 (Bartlett) More than fifty years ago, Mr. Samuel Adams's father, and twenty others‥used to meet, make a caucus, and lay their plan for introducing certain persons into places of trust and power. 1809 Kendall Trav. I. xv. 174 A caucus is a political, and what is in practice the same thing, a party meeting; but it is not a popular meeting‥It is in caucuses that it is decided, for whom the people shall be instructed to vote, and by what course of politics the party may be secured. 1818 Syd. Smith Wks. (1869) 271 Caucus, the cant word of the Americans. 1847 in Craig. a1850 J. C. Calhoun Wks. I. 41 Party organization, and party caucuses. 1853 Lytton My Novel xii. xii. (D.), I think of taking a hint from‥America, and establishing secret caucuses: nothing like 'em. 1855 Motley Dutch Rep. I. 360 The meeting was, in fact, what we should call a caucus, rather than a general gathering. 1870 Lowell Among my Bks. Ser. ii. (1873) 98 In the Greek epic, the gods are partisans, they hold caucuses, they lobby and log-roll for their candidates.

b. in reference to other countries.

1886 New Zealand Her. 1 June 4/5 The Auckland members are to have a caucus this morning, to consider what action should be taken in connection with the appropriation.

2. In English newspapers since 1878, generally misused, and applied opprobriously to a committee or organization charged with seeking to manage the elections and dictate to the constituencies, but which is, in fact, usually a representative committee popularly elected for the purpose of securing concerted political action in a constituency.
It was first applied in 1878, by Lord Beaconsfield and the Times newspaper, to the organization of the Birmingham Liberal ‘Six Hundred’, and thence to those which were speedily formed on its model elsewhere; the implication being that this was an introduction of ‘the American system’ into English politics, which deserved to be branded with an American name. But the name was grotesquely misapplied: in American use, a caucus is a meeting; English newspapers apply the caucus to an organization or system. Such organizations have since been, in one form or another, adopted by all parties; and caucus is now a term which partizans fling at the organizations of their opponents, and disclaim for their own.

1878 Times 31 July 10/4 The policy of the politicians of the Midland capital will bring upon us the ‘caucus’ with all its evils‥The introduction and progress of the ‘caucus’ system among us. 1878 J. Chamberlain Let. in ibid. 1 Aug. 8/1 The ‘Birmingham system’‥I observe that you, in common with the Prime Minister [Ld. Beaconsfield] have adopted the word ‘caucus’ to designate our organization. 1878 in ibid., (Correspondent), To secure their election as members of a ‘caucus’. 1879 Times 30 Jan. 9/2 The Southwark Caucus. 1882 Sat. Rev. 29 Apr., When he has made it impossible for any man to obtain a seat in Parliament except by dint of the Caucus. Ibid. 27 May, ‘Government by Caucus.’ The English Liberal Associations with their ruling committees‥ Ibid. 21 Oct., Mr. Davitt represents Communism and the Caucus. 1883 Birmingh. Weekly Post 14 Apr. 4/7 A history of the Tory Caucus‥would go a long way back. 1884 Fortn. Rev. June 831 The Caucus, as it is now adopted by the Tories, is a species of organization fundamentally different from that‥employed by the Liberals.

3. attrib. and comb.

1763 [see 1]. 1879 Campbell White & Bl. in U.S. 63 The caucus system which prevails in America in regard to elections. 1878 Black Green Past. xvi. 128 He was accustomed to much recondite diplomacy, caucus meetings, private influence. 1882 Q. Rev. Oct. 56 The Caucus-mongers have not given any reflection to this point. 1884 Sat. Rev. 5 July 9/1 Whatever the real feeling‥it is‥obvious that the Caucus-mongers are going the precise way to obscure it.