Not Long For This World Meaning

Example : 1787, John Quincy Adams, Charles Francis Adams, editor, Life in a New England Town, 1787, 1788: Diary of John Quincy Adams, (1903) page 109:  Townsend went there with us, but found himself so unwell that he went home very early. His cough has return'd, with several disagreeable symptoms. I fear exceedingly that he is not long for this world.1843 Thomas Carlyle, Past and Present, book 1, ch. 6, Hero-Worship:  He thinks that said soul will have to be resuscitated from its asphyxia; that if it prove irresuscitable, the man is not long for this world.1928, Dorothy L. Sayers, "The Undignified Melodrama of the Bone of Contention", in Dorothy L. Sayers: the complete stories, (2002) page 92:  But I've had a warning, and I'm not long for this world."  "Not long for this world? Oh, nonsense, Plunkett. You mustn't talk like that. A touch of indigestion, that's what you've got, I expect.2006, Richard Peck, The Teacher's Funeral: A Comedy in Three Parts, page 188:  Though Aunt Maud had always maintained she was not long for this world, she outlived all her generation.