Etymology
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jack-knife (v.)
1776, "to stab," from jack-knife (n.). Intransitive meaning "to fold or bend" the body is said to date from the time of the American Civil War. The truck accident verbal sense is from 1949. Related: Jackknifed; jackknifing.
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cracker-jack (n.)

also crackerjack, "something excellent," 1893, U.S. colloquialism, apparently a fanciful construction, earliest use in reference to racing horses. The caramel-coated popcorn-and-peanuts confection was said to have been introduced at the World's Columbian Exposition (1893). Supposedly a salesman gave it the name when he tasted some and said, "That's a cracker-jack," using the then-popular expression. The name was trademarked 1896. The "Prize in Every Box" was introduced 1912.

"Your brother Bob is traveling, isn't he?"
"Yep. He's with one of the big racing teams. I tell you, he's a cracker-jack! Wins a bushel of diamonds and gold cups every week."
[Life magazine, Aug. 1, 1895]
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Jill 
fem. proper name, Middle English Jille, Jylle, Gille, etc., familiar shortening of Jillian, Gillian, which represent the common Middle English pronunciation of Juliana (see Gillian). A very popular name for girls in medieval England, hence its use as a familiar, almost generic, name for a girl (early 15c.; paired with Jack since mid-15c.).
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hijack (v.)
by 1922 (perhaps c. 1918), American English, of unknown origin; perhaps from high(way) + jacker "one who holds up" (agent noun from jack (v.)). In early use "to rob (a bootlegger, smuggler, etc.) in transit;" sense of "seize an aircraft in flight" is 1968 (also in 1961 variant skyjack), extended 1970s to any form of public transportation. Related: Hijacked; hijacking. Related: Hijacker.
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Jenny 

fem. personal name, originally another form of Jane, Janey and a diminutive of Jane or Janet; in modern use (mid-20c.) typically a shortening of Jennifer. Jenny is attested from c. 1600 as female equivalent of jack (n.), and like it applied to animals (especially of birds, of a heron, a jay, but especially Jenny wren, 1640s, in bird-fables the consort of Robin Redbreast). Also like jack used of machinery; Akrwright's spinning jenny (1783) is said to have been named for his wife, but is perhaps rather a corruption of gin (n.2) "engine."

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jacket (n.)
mid-15c., "short garment for men," from Old French jaquet "short coat with sleeves," diminutive of jaque, a kind of tunic, which is of uncertain origin. Probably it is from Jacque, the male proper name, also the generic name of a French peasant (see jacquerie) with extended material senses as in native jack (n.). But possibly it is from or influenced by jaque (de mailles) "short, tight-fitting coat," originally "coat of mail," from Spanish jaco, from Arabic shakk "breastplate." Meaning "paper wrapper of a book" is first attested 1886.

Iakke, jakke "a short, close-fitting stuffed or quilted tunic, often serving as a defensive garment" is attested in English from late 14c. (from Old French jaque), and by c. 1400 was being used for "woman's short tunic." It is possible that jacket was formed in English as a diminutive of this.
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jackpot (n.)

also jack-pot, "big prize," 1944, from slot machine sense (1932), from now-obsolete poker sense (1881) in reference to antes that begin when no player has a pair of jacks or better; from jack (n.) in the card-playing sense + pot (n.1). Earlier, in criminal slang, it meant "trouble," especially "an arrest" (1902).

The regular Draw-Poker game is usually varied by occasional Jack-Pots, which are played once in so many deals, or when all have refused to play, or when the player deals who holds the buck, a marker placed in the pool with every jack-pot. In a jack-pot each player puts up an equal stake and receives a hand. The pot must then be opened by a player holding a hand of the value of a pair of knaves (jacks) or better. If no player holds so valuable a hand the deal passes and each player adds a small sum to the pot or pool. When the pot is opened the opener does so by putting up any sum he chooses, within the limit, and his companions must pay in the same amount or "drop." They also possess the right to raise the opener. The new cards called for are then dealt and the opener starts the betting, the play proceeding as in the regular game. [Encyclopaedia Britannica, 11th ed., 1911, "Poker." The article notes "Jack-Pots were introduced about 1870."]

To hit the jackpot "be very successful" is from 1938.

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Jacksonian 
1824, of or in the character of U.S. politician Andrew Jackson (1767-1845). The surname is recorded from early 14c., literally "Jack's son, son of a man named Jack." Jacksonville, Florida, was renamed for him in 1822 from earlier Cowford, said to be an English translation of a native name wacca pilatka [Room].
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fee-faw-fum (interj.)
exclamation of the giant in "Jack the Giant-Killer," c. 1600.
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bean-stalk (n.)
also beanstalk, "stem of a bean plant," 1800 (in the story of Jack and the giant), from bean (n.) + stalk (n.).
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