Wiktionary
not long for this world
a. (context idiomatic English) unlikely to endure for much more time. alt. (context idiomatic English) unlikely to endure for much more time.
Wikipedia
Not Long for this World
Not Long for this World is a collection of fantasy and horror short stories by author August Derleth. It was released in 1948 and was the author's third collection published by Arkham House. It was released in an edition of 2,067 copies.
Most of the stories had originally appeared in the magazine Weird Tales.