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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song (n.)

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.

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plain-song (n.)

also plainsong, unisonous vocal music used in the Christian churches in the earliest centuries, mid-15c., translating Latin cantus planus, French plain chant; see plain (adj.) + song (n.).

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sing-song (adj.)
also singsong, musically repetitive and unvarying, 1734, from earlier use as a noun meaning "a jingling ballad" (c. 1600), from sing (v.) + song (n.).
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part-song (n.)

"vocal composition for two or more independent voices," usually sung without accompaniment, by 1824, from part (n.) in the musical sense + song (n.).

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love-song (n.)
early 14c., from love (n.) + song (n.).
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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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canticle (n.)
"short hymn," early 13c., from Latin canticulum "a little song," diminutive of canticum "song" (also a scene in Roman comedy enacted by one person and accompanied by music and dancing), from cantus "song, a singing; bird-song," from past participle stem of canere "to sing" (from PIE root *kan- "to sing").
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