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knuckle [knuckle 词源字典]: knuckle: [14] Knuckle originally denoted the rounded end of a bone at a joint, which sticks out when you bend the joint. This could be at any joint, including the elbow, the knee and even the joints of the vertebrae; only gradually did it become specialized to the finger joints. The word probably came from Middle Low German knökel (or a relative of it), which appears to have meant etymologically ‘little bone’. Knuckle down, in the sense ‘begin to work hard and conscientiously’, comes from the game of marbles, where players have to put their knuckles on the ground when shooting a marble with the thumb.
[knuckle etymology, knuckle origin, 英语词源]
conscientious (adj.): 1610s, from Middle French conscientieux (16c.; Modern French consciencieux), from Medieval Latin conscientiosus, from conscientia (see conscience). Related: Conscientiously; conscientiousness.
jumbo (adj.): "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. 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).
"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]
As a product size, by 1886 (cigars). Jumbo jet attested by 1964.