Etymology
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green (n.)
late Old English, "green color or pigment, spectral color between blue and yellow;" also "a field, grassy place; green garments; green foliage," from green (adj.). Specific sense "piece of grassland in a village belonging to the community" is by late 15c. In golf, "the putting portion of the links" by 1849. Symbolic of inconstancy since late 14c., perhaps because in nature it changes or fades. Also symbolic of envy and jealousy since Middle English. Shakespeare's green-eyed monster of "Othello" sees all through eyes tinged with jealousy. "Greensleeves," ballad of an inconstant lady-love, is from 1570s. The color of the cloth in royal counting houses from late 14c., later the color of the cloth on gambling tables.
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missing (adj.)

"not present or found, absent," 1520s, present-participle adjective from miss (v.). Military sense of "not present after a battle but not known to have been killed or captured" is from 1845. As a noun by 1855.

Missing link was used in various figurative senses before is attested 1846 in reference to forms of plant and animal life [Chambers, "Vestiges of the Natural History of Creation"]; in reference to a hypothetical creature between man and the apes, as a component of Darwin's theory of evolution, by 1860.

But then, where are the missing links in the chain of intellectual and moral being? What has become of the aspirants to the dignity of manhood whose development was unhappily arrested at intermediate points between the man and the monkey? It will not be doubted, we presume, that there exists at present an enormous gap between the intellectual capabilities of the lowest race of men and those of the highest race of apes; and if so, we ask again, why should the creatures intermediate to them—exalted apes or degraded men—have been totally exterminated, while their less worthy ancestors have successfully struggled through the battle of life? [William Hopkins, "Physical Theories of the Phenomena of Life," Fraser's Magazine, July, 1860]

In a popular religious tract of the late 1850s, "The Missing Link of the London Poor," the missing link was the Bible. Missing person, one who has disappeared and whose condition, whether alive or dead, is unknown, is by 1820.

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chain (n.)

c. 1300, "connected series of links of metal or other material," from Old French chaeine "chain" (12c., Modern French chane), from Latin catena "chain" (source also of Spanish cadena, Italian catena), which is of unknown origin, perhaps from a PIE root *kat- "to twist, twine" (source also of Latin cassis "hunting net, snare").

As a type of ornament worn about the neck, from late 14c.  As a linear measure ("a chain's length") from 1660s. From 1590s as "any series of things linked together." Meaning "a series of stores controlled by one owner or firm" is American English, 1846. Figurative use "that which binds or confines" is from c. 1600.

Chain-reaction is from 1916 in physics, specific nuclear physics sense is from 1938; chain-mail armor is from 1795, from mail (n.2). Before that, mail alone sufficed. Chain letter is recorded from 1892; at first usually to raise money; decried from the start as a nuisance.

Nine out of every ten givers are reluctant and unwilling, and are coerced into giving through the awful fear of "breaking the chain," so that the spirit of charity is woefully absent. [St. Nicholas magazine, vol. xxvi, April 1899] 

Chain of command is from 1915. Chain-lightning, visible as jagged or broken lines, is from 1834. Chain-smoker, one who smokes one after another, lighting the next from the stump of the last, is attested from 1885, originally of Bismarck (who smoked cigars), thus probably a loan-translation of German Kettenraucher. Chain-smoking (n.) is from 1895.

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secular (adj.)
Origin and meaning of secular

c. 1300, seculer, in reference to clergy, "living in the world, not belonging to a religious order," also generally, "belonging to the state" (as opposed to the Church), from Old French seculer, seculare (Modern French séculier) and directly from Late Latin saecularis "worldly, secular, pertaining to a generation or age," in classical Latin "of or belonging to an age, occurring once in an age," from saeculum "age, span of time, lifetime, generation, breed."

This is from Proto-Italic *sai-tlo-, which, according to Watkins, is PIE instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. De Vaan also connects it with "bind" words and lists as a cognate Welsh hoedl "lifespan, age." An older theory connected it to words for "seed," from PIE root *se- "to sow" (see sow (v.), and compare Gothic mana-seþs "mankind, world," literally "seed of men").

The ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years).  Ecclesiastical writers in Latin used it as those in Greek did aiōn "of this world" (see cosmos). It is the source of French siècle "century." The meaning "of or belonging to an age or a long period," especially occurring once in a century, was in English from 1590s.

From mid-14c. in the general sense of "of or belonging to the world, concerned in earthly more than in spiritual, life;" also of literature, music, etc., "not overtly religious." In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s. Related: Secularly.

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