a swan song

     Some kinds of swans ( large birds, something like geese ) supposedly only sing once in lifetime--just before they die.  So to refer to something as someone's "swan song" is to say that it was the only speech they made before they died. To tell someone that a speech will be their "swan song" is to tell them the words will be their last--to threaten to "off" them, or to have them "offed", mobster-style.