Etymology
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black market (n.)
"unauthorized dealing in restricted or rationed commodities," 1931, from black (adj.), probably suggesting "dark, invisible" or "shady, improper" + market (n.). As an adjective by 1935. It exploded in popularity with the coming of World War II rationing.
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Black Hand (n.)
Italian immigrant secret society in U.S., 1904; earlier a Spanish anarchist society, both from the warning mark they displayed to potential victims.
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black widow (n.)

type of poisonous spider (Latrodectus mactans) in U.S. South, 1904, so called from its color and from the female's supposed habit of eating the male after mating (the males seem to get eaten more often before they mate, when they first enter the webs of the females, which have very poor eyesight). Sometimes also known as shoe-button spider. The name black widow is attested earlier (1830s) as a translation of a name of the "scorpion spider" of Central Asia.

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black-tie (n.)
as an article of male attire, 1848, from black (adj.) + tie (n.). As an adjective, indicating the style of formal attire that features it, or situations where such is the proper dress, by 1933.
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coal-black (adj.)

"black as coal," mid-13c., from coal (n.) + black (adj.).

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rara avis (n.)

c. 1600, "peculiar person, person of a type seldom encountered," from Latin rara avis, literally "strange bird," from rara, fem. of rarus "rare" (see rare (adj.1)) + avis "bird" (see aviary). Latin plural is raræ aves. A phrase used of Horace's peacock (a Roman delicacy), Juvenal's black swan ("Rara avis in terris, nigroque simillima cygno"). A figure perhaps natural to the superstitious Romans, who divined by bird-watching.

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Deneb 

bright star in the tail of the constellation Cygnus the Swan, by 1741, from Arabic Al Dhanab al Dajajah "the Hen's Tail."

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*swen- 

also swenə-, Proto-Indo-European root meaning "to sound." 

It forms all or part of: assonance; consonant; dissonant; resound; sonant; sonata; sone; sonic; sonnet; sonogram; sonorous; sound (n.1) "noise, what is heard;" sound (v.1) "to be audible;" swan; unison.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit svanati "it sounds," svanah "sound, tone;" Latin sonus "sound, a noise," sonare "to sound;" Old Irish senim "the playing of an instrument;" Old English geswin "music, song," swinsian "to sing;" Old Norse svanr, Old English swan "swan," properly "the sounding bird."

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alb (n.)
late Old English albe "white linen robe" worn by priests, converts, etc., from Late Latin alba (in tunica alba or vestis alba "white vestment"), fem. of albus "white," from PIE root *albho- "white" (source also of Greek alphos "white leprosy," alphiton "barley meal;" Old High German albiz, Old English elfet "swan," literally "the white bird;" Old Church Slavonic and Russian lebedi, Polish łabędź "swan;" Hittite alpash "cloud").
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cygnet (n.)

"a young swan," c. 1400, also signet before 17c., from Anglo-French cignet (mid-14c.), Anglo-Latin cygnettus, diminutives of Old French cigne, cisne "swan" (12c., Modern French cygne), from Latin cygnus, from Greek kyknos, which has been the subject of "abundant discussion" (Beekes) and is perhaps from PIE *(s)keuk- "to be white" (compare Sanskrit socati "to lighten, glow," sukra- "light, clear, white"). Spanish, Portuguese cisne, Italian cecero are from Medieval Latin cecinus, cicinus, a corruption of the classical Latin word.

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