Etymology
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upbeat (adj.)

"with a positive mood," 1947, apparently from on the upbeat "improving, getting better," attested from 1934 and a favorite of Billboard magazine headline-writers in the early 1940s, from the musical noun upbeat (1869), referring to the beat of a bar at which the conductor's baton is in a raised position; from up (adv.) + beat (n.). The "optimistic" sense apparently for no other reason than that it sounds like a happy word (the musical upbeat is no more inherently "positive" than any other beat).

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fain (adj.)

Old English fægen, fagen "glad, cheerful, happy, joyful, rejoicing," from a common Germanic root (cognates: Old Saxon fagan, Old Norse feginn "glad," Old High German faginon, Gothic faginon "to rejoice"), perhaps from PIE *pek- (1) "to make pretty." Often it means "glad" in a relative sense, "content to accept when something better is unobtainable." As an adverb, from c. 1200. Related: Fainly. Compare fawn (v.).

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felicity (n.)

late 14c., "happiness; that which is a source of happiness," from Old French felicite "happiness" (14c.), from Latin felicitatem (nominative felicitas) "happiness, fertility," from felix (genitive felicis) "happy, fortunate, fruitful, fertile" (from suffixed form of PIE *dhe(i)- "to suck," with derivatives meaning "to suckle, produce, yield").

A relic of Rome's origins as an agricultural community: that which brings happiness is that which produces crops. Compare pauper (see poor (adj.)) "poor, not wealthy," literally "producing little." The meaning "skillful adroitness, admirable propriety" is attested from c. 1600.

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Letitia 
fem. proper name, literally "gladness," from Latin laetitia "joy, exultation, rejoicing, gladness, pleasure, delight," from laetus "glad, happy; flourishing, rich," a word of unknown origin. On the assumption that "fat, rich" is the older meaning, this word has been connected to lardus "bacon" and largus "generous," but de Vaan finds this "a very artificial reconstruction." In 17c. English had a verb letificate "make joyful" (1620s), and Middle English had letification "action of rejoicing" (late 15c.).
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red-blooded (adj.)

1802, "having red or reddish blood," from red (adj.1) + blood (n.). "Specifically noting the higher worms, or annelids, in which, however, the blood is often greenish" [Century Dictionary]. The figurative meaning "vigorous, spirited" is recorded by 1862.

The children born in California are certainly a great improvement upon those born among us. Nowhere can more rosy specimens of health and beauty be found. Strong-limbed, red-blooded, graceful, and as full of happy animal life as young fawns, they bid fair to develop into admirable types of manhood and womanhood. [Bayard Taylor, "New Pictures from California" 1862]
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Callisto 

in classical mythology a nymph, mother of Arcas by Zeus, turned to a bear by Hera, from Greek kallistos, superlative of kalos "beautiful, beauteous, noble, good," and its derived noun kallos "beauty," from *kal-wo-, which is of uncertain origin, perhaps related to Sanskrit kalyana "beautiful." The usual combining form in Greek was kalli- "beautiful, fine, happy, favorable;" kalo- was a later, rarer alternative form. Also a name given to the fourth moon of Jupiter in 17c. but not widely used before mid-19c. Feminized as proper name Callista.

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fecund (adj.)

a 16c. Latinizing revision of the spelling of Middle English fecond, fecound (early 15c.), from Old French fecond, fecont "fruitful" and directly from Latin fecundus "fruitful, fertile, productive; rich, abundant," from *fe-kwondo-, suffixed form (adjectival) of PIE root *dhe(i)- "to suck, suckle," with derivatives meaning also "produce, yield."

Also from the same Latin root come felare "to suck;" femina "woman" (literally "she who suckles"); felix "happy, auspicious, fruitful;" fetus "offspring, pregnancy;" fenum "hay" (probably literally "produce"); and probably filia/filius "daughter/son," assimilated from *felios, originally "a suckling."

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jay (n.)

the common European jay (Garrulus glandarinus), early 14c. (late 12c. as a surname), from Old North French gai, Old French jai "magpie, jay" (12c., Modern French geai), from Late Latin gaius "a jay," probably echoic of the bird's harsh warning cry and supposedly influenced by Latin Gaius, a common Roman proper name.

For other bird names from proper names, compare martin and parrot. Applied to the North American blue jay (Cyanocitta cristata) from 1709; it is unrelated but has similar vivid markings, is noisy and restless, and also has a harsh call. Applied to humans in sense of "impertinent chatterer, loud, flashy dresser" from 1520s. Jolly as a jay was a Middle English expression for "very happy, joyful."

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holiday (n.)
1500s, earlier haliday (c. 1200), from Old English haligdæg "holy day, consecrated day, religious anniversary; Sabbath," from halig "holy" (see holy) + dæg "day" (see day); in 14c. meaning both "religious festival" and "day of exemption from labor and recreation," but pronunciation and sense diverged 16c. As an adjective mid-15c. Happy holidays is from mid-19c., in British English, with reference to summer vacation from school. As a Christmastime greeting, by 1937, American English, in Camel cigarette ads.
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Scilly 

isles off Cornwall, of unknown origin. Pliny has Silumnus, Silimnis. Perhaps it is connected with the Roman god Sulis (compare Aquae sulis "Bath"). The -y might be Old Norse ey "island." The -c- was added 16c.-17c. "[A]bout the only certain thing that can be said is that the c of the modern spelling is not original but was added for distinction from ModE silly as this word developed in meaning from 'happy, blissful' to 'foolish.'" ["Cambridge Dictionary of English Place-Names"].

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