The word swan has descended unchanged from the Old Saxon word swan or suan. The Old English and Middle English forms were also swan. The Old Saxon form derived from the Proto-Germanic *swanaz. The Old High German forms were swan, swan(a) and swon, the Old Norse form was svanr, and the Middle Dutch form was swane.
The English word is cognate with the Danish svane, Dutch zwaan, German schwan, Icelandic svanr and Swedish svan.
These words all derive from the Indo-European root *swen or *swon (to sound, to sing).
The Latin word sonus (sound) and the Sanskrit word svan (to sound, to sing) have the same root.
The Classical Greek κυκνωσ (kuknōs) has a different etymology. From it derives the Latin cygnus, whence the French cygne, Italian cigno, Portuguese cisne, Spanish cisne, and similar forms in other Romance languages. The Hebrew ספרותי and Russian лебедь also have different origins.
The term “swan song“, first attested in 1831, is a translation of the German Schwanengesang. A black swan was proverbial for “something extremely rare or non-existent” (1398), after Juvenal (Satires 6:164). The term “swan dive” dates from 1898.