swan song

Swans don't sing. They whistle or trumpet, or in the case of the swan most common in ponds, the mute swan, they only hiss and snort. But according to ancient legend, the swan does sing one beautiful song in its life-just before it dies. References in English to the dying swan's lovely singing go back as far as Chaucer, but the term swan song itself didn't appear in the language until the 1830s, when Thomas Carlyle used it in Sartor Resartus. Carlyle probably based his "swan song" on the German version of the term, which is Schwanengesang or Schwanenlied.