Etymology
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jackaroo (n.)
Australian for "a new arrival from Britain," 1867, from Jack + ending from kangaroo. The female counterpart jillaroo is attested from 1945.
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applejack (n.)
also apple-jack, "apple-brandy, liquor distilled from cider," 1816, from apple + jack (n.).
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steeplejack (n.)
"one who climbs steeples, chimneys, etc. to make repairs," 1881, from steeple + jack (n.) "fellow, man."
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lumberjack (n.)
1831, Canadian English, from lumber (n.) + jack (n.) "man, fellow." Lumberman in the same sense is from 1817.
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jackass (n.)

"male ass," 1727, from jack (n.) + ass (n.1). Contemptuous meaning "stupid person" is attested by 1784 (Ignatius Sancho). Related: Jackassism (1837, American English); jackassery (1833).

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jackstraw (n.)
1590s, "effigy of a man made of straw," from Jack + straw (n.); hence "man without substance or means." It also was a name of one of the leaders of the Peasants' Revolt of 1381. As the name of a game played with straw or strips, from 1801. Related: Jackstraws.
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jackanapes (n.)

mid-15c., "a monkey," also "an impertinent, conceited fellow, an absurd fop," a general term of reproach (in mid-15c. especially a contemptuous nickname for William de la Pole, Duke of Suffolk), of unknown origin. Apparently from Jack of Naples, but whether this is some specific personification of Jack (which is attested from 16c. as "saucy or impertinent fellow") or folk etymology of jack (n.) + ape (n.) is unknown. See extensive note in OED. Century Dictionary suggests "orig., it is supposed, a man who exhibited performing apes." Farmer and Henley ("Slang and Its Analogues") say "originally, no doubt, a gaudy-suited and performing ape." Its fem. counterpart is Jane-of-apes (Massinger) "a pert, forward girl."

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flapjack (n.)
pre-1600, from flap (v.) + jack (n.), using the personal name in its "generic object" sense. So called from the process of baking it by flipping and catching it in the griddle when done on one side.
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jackboot (n.)
also jack-boot, 1680s, type of large, strong over-the-knee cavalry boot of 17c.-18c., later a type worn by German military and para-military units in the Nazi period. From jack (n.), though the exact sense here is unclear + boot (n.1). Figurative of military oppression since 1768. Related: Jackbooted.
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jimmy (n.)
"burglar's crowbar," 1848, variant of jemmy, name for a type of crowbar much used by burglars, special use of Jemmy, familiar form of proper name James (compare the mechanical uses of jack (n.)).
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