"very large, unusually large for its type," 1882, a reference to Jumbo, name of the London Zoo's huge elephant (acquired from France, said to have been captured as a baby in Abyssinia in 1861), sold February 1882 to U.S. circus showman P.T. Barnum amid great excitement in America and great outcry in England, both fanned by Barnum.
"I tell you conscientiously that no idea of the immensity of the animal can be formed. It is a fact that he is simply beyond comparison. The largest elephants I ever saw are mere dwarfs by the side of Jumbo." [P.T. Barnum, interview, "Philadelphia Press," April 22, 1882]
The name is perhaps from slang jumbo "clumsy, unwieldy fellow" (1823), which itself is possibly from a word for "elephant" in a West African language (compare Kongo nzamba). OED suggests it is possibly the second element in Mumbo Jumbo. Century Dictionary says "The name was given as having an African semblance." As a product size, by 1886 (cigars). Jumbo jet attested by 1964. Jumbo was accidentally killed near St. Thomas, Ontario, Sept. 15, 1885, struck by a freight train while the circus was loading up to travel.