Online Etymology Dictionary: Old English swan, from Proto-Germanic *swanaz (cf. Old Saxon swan, Old Norse svanr, Middle Dutch swane, Dutch zwaan, Old High German swan, German Schwan), probably lit. “the singing bird,” from Proto-Indo-European base *swon-/*swen- “to sing, make sound” (see sound (n.1)); thus 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. A black swan was proverbial for “something extremely rare or non-existent” (late 14c.), after Juvenal [Satire vi. 164].

“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 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.

Swan dive is recorded from 1898.

Online Etymology Dictionary: swain: mid-12 century, “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 Proto-Indo-European *swoi-no-, from base *swe- “oneself, alone, apart.”

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.

boatswain: mid-15c., from late O.E. batswegen, from bat “boat” + O.N. sveinn “boy.” Phonetic spelling bo’sun is attested from 1868.

[rom the Elder or Poetic Edda. Auden translation]

For these things give thanks at nightfall:
The day gone, a guttered torch,
A sword tested, the troth of a maid,
Ice crossed, ale drunk.

Hew wood in wind-time,
in fine weather sail,
Tell in the night-time tales to house-girls,
For too many eyes are open by day:
From a ship expect speed, from a shield, cover,
Keenness from a sword,
but a kiss from a girl.

Drink ale by the hearth, over ice glide,
Buy a stained sword, buy a starving mare
To fatten at home: and fatten the watch-dog.

Trust not an acre early sown,
Nor praise a son too soon:
Weather rules the acre, wit the son,
Both are exposed to peril,

A snapping bow, a burning flame,
A grinning wolf, a grunting boar,
A raucous crow, a rootless tree,
A breaking wave, a boiling kettle,
A flying arrow, an ebbing tide,
A coiled adder, the ice of a night,
A bride’s bed talk, a broad sword,
A bear’s play, a prince’ s children,
A witch’ s welcome, the wit of a slave,
A sick calf, a corpse still fresh,
A brother’s killer encountered upon
The highway, a house half-burned,
A racing stallion who has wrenched a leg,
Are never safe: let no man trust them.

Maître Corbeau, sur un arbre perché,
Tenait en son bec un fromage.
Maître Renard, par l’odeur alléché,
Lui tint à peu près ce langage :
« Et Bonjour, Monsieur du Corbeau.
Que vous êtes joli ! que vous me semblez beau !
Sans mentir, si votre ramage,
Se rapporte à votre plumage,
Vous êtes le Phénix des hôtes de ces bois. »
A ces mots, le corbeau ne se sent pas de joie ;
Et pour montrer sa belle voix,
Il ouvre un large bec, laisse tomber sa proie.
Le renard s’en saisit, et dit : « Mon bon Monsieur,
Apprenez que tout flatteur
Vit aux dépens de celui qui l’écoute.
Cette leçon vaut bien un fromage, sans doute. »
Le corbeau, honteux et confus,
Jura, mais un peu tard, qu’on ne l’y prendrait plus.