The OED has the same with a little more:
[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 cau´-cau-as´u, 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.]