Etymology
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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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bitter end (n.)

In lexicons of sea language going back to 1759, the bitter end is the part of a cable which is round about the bitts (the two great timbers used to belay cables) when the ship is at anchor (see bitt).

Bitter end of the Cable, the End which is wound about the Bitts. ["The News-Readers Pocket-Book: Or, a Military Dictionary," London, 1759]

So, when a cable is played out to the bitter end, there is no more left to play. The term began to be used c. 1835 in non-nautical use and with probable influence of or merger with bitter (adj.).

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bee's knees (n.)
1923, a survivor of a fad around this year for slang terms denoting "excellence" and based on animal anatomy. Also existed in the more ribald form bee's nuts. Other versions that lasted through the century are cat's whiskers (1923), cat's pajamas, cat's meow. More obscure examples are canary's tusks, cat's nuts and flea's eyebrows. The fad still had a heartbeat in Britain at the end of the century, as attested by the appearance of dog's bollocks in 1989. Bee's knee was used as far back as 1797 for "something insignificant."
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guinea pig (n.)
rodent native to South America, 1660s. It does not come from Guinea and has nothing to do with the pig. Perhaps so called either because it was brought back to Britain aboard Guinea-men, ships that plied the triangle trade between England, Guinea, and South America [Barnhart, Klein], or from its resemblance to the young of the Guinea-hog "river pig" [OED], or from confusion of Guinea with the South American region of Guyana (but OED is against this). Pig probably for its grunting noises. In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in medical experiments (by 1865).
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