This term is so overused by sportscasters who don't even know what it means. I was surprised to learn myself:
• Etymology Today from M-W: swan song
1 : a song of great sweetness said to be sung by a dying swan
*2 : a farewell appearance or final act or pronouncement

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."

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