Etymology
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jai alai (n.)
1902, American English, originally in a Cuban context, from Basque, from jai "celebration" + alai "merry."
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Jazz Age 

1921; see jazz (n.); popularized 1922 in writings of F. Scott Fitzgerald; usually regarded as the years between the end of World War I (1918) and the Stock Market crash of 1929.

We are living in a jazz age of super-accentuated rhythm in all things; in a rhythm that (to "jazz" a word) is super-normal, a rhythm which is the back-flare from the rhythm of a super war. ["Jacobs' Band Monthly," Jan. 1921]
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je ne sais quoi (n.)

"an inexpressible something," French, literally "I do not know what."

[T]hey are troubled with the je-ne-scay-quoy, that faign themselves sick out of niceness but know not where their own grief lies, or what ayls them. [Thomas Blount, "Glossographia," 1656]
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jerk off (v.)

slang, "perform male masturbation," by 1896, from jerk (v.) denoting rapid pulling motion + off (adv.). Compare come off "experience orgasm" (17c.). Farmer and Henley ("Slang and Its Analogues") also lists as synonyms jerk (one's) jelly and jerk (one's) juice. The noun jerk off or jerkoff as an emphatic form of jerk (n.2) is attested by 1968. As an adjective from 1957.

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jet lag (n.)
also jetlag, 1966, from jet (n.1) in the "airplane" sense + lag (n.). Also known in early days as time zone syndrome.
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jeu d'esprit (n.)
"a witticism," 1712, from French, from jeu "play, game," from Latin jocum "jest, joke, play, sport" (see joke (n.)).
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jeune fille (n.)
1802, French, literally "young girl," from jeune "young," from Latin juvenis (see young (adj.)).
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jeunesse doree (n.)
1811, French jeunesse dorée "gilded youth, rich and fashionable young men," from jeunesse "youth," from jeune "young" (12c.), from Latin iuvenis "young man" (see young (adj.)) + fem. of doré "gilded."
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John Q. Public (n.)
"imaginary average American citizen," 1934; the Q perhaps suggested by John Quincy Adams.
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joie de vivre (n.)
1889, French, literally "joy of living."
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