Etymology
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swan (n.)

Old English swan "swan," from Proto-Germanic *swanaz "singer" (source also of Old Saxon swan, Old Norse svanr, Danish svane, Swedish svan, Middle Dutch swane, Dutch zwaan, Old High German swan, German Schwan), probably literally "the singing bird" (from PIE root *swen- "to make sound"). If so, it is related to Old English geswin "melody, song" and swinsian "to make melody."

In classical mythology, sacred to Apollo and to Venus. The singing of swans before death was alluded to by Chaucer (late 14c.), but swan-song (1831) is a translation of German Schwanengesang. The ancient Indo-European mythical swan-maiden so called by mythographers from 1829. Swan dive is recorded from 1898.

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black swan (n.)

proverbial for "something extremely rare or non-existent" (late 14c.) is from Juvenal ["Sat." vi. 164], but the real thing turned up in Australia (Chenopsis atratus).

"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]

Blue dahlia also was used 19c. for "something rare and unheard of."

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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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Castor 

one of the divine twins (brother of Pollux), also the name of the alpha star of Gemini, Latin, from Greek Kastor, perhaps literally "he who excels." They were sons of Tyndarus, king of Sparta (but in post-Homeric myth of Zeus in the form of a swan)  and Leda.

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auk (n.)
type of large, colonial diving bird, 1670s, a Northern England name, from a Scandinavian source such as Old Norse alka, probably originally imitative of a water-bird cry (compare Latin olor "swan," Greek elea "marsh bird"). Originally and properly the great auk, which once abounded on North Atlantic coasts; hunted for food, bait, and its down, the species became extinct c. 1850.
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swain (n.)
mid-12c., "young man attendant upon a knight," from Old Norse sveinn "boy, servant, attendant," from Proto-Germanic *swainaz "attendant, servant," properly "one's own (man)," from PIE *swoi-no-, from root *s(w)e- "oneself, alone, apart" (see idiom). Cognate with Old English swan "shepherd, swineherd," Old Saxon swen, Old High German swein. Meaning "country or farm laborer" is from 1570s; that of "lover, wooer" (in pastoral poetry) is from 1580s.
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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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