"For whom the bell tolls" is a line from a poem by John Donne (pronounced like "Dunn") written in the early 1600s. Hemingway used a line from the poem as the title of a novel he wrote in the 20th century.
The poem goes like this (the copyright is in the public domain):
No man is an island,
Entire of itself.
Each is a piece of the continent,
A part of the main.
If a clod be washed away by the sea,
Europe is the less.
As well as if a promontory were.
As well as if a manor of thine own
Or of thine friend's were.
Each man's death diminishes me,
For I am involved in mankind.
Therefore, send not to know
For whom the bell tolls,
It tolls for thee.
