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.
Old English sang "voice, song, art of singing; metrical composition adapted for singing, psalm, poem," from Proto-Germanic *songwho- (source also of Old Norse söngr, Norwegian song, Swedish sång, Old Saxon, Danish, Old Frisian, Old High German, German sang, Middle Dutch sanc, Dutch zang, Gothic saggws), from PIE *songwh-o- "singing, song," from *sengwh- "to sing, make an incantation" (see sing (v.)).
Phrase for a song "for a trifle, for little or nothing" is from "All's Well" III.ii.9 (the identical image, por du son, is in Old French. With a song in (one's) heart "feeling joy" is first attested 1930 in Lorenz Hart's lyric. Song and dance as a form of vaudeville act is attested from 1872; figurative sense of "rigmarole" is from 1895.
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."
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."