The deep, slow sound of a bell, especially when rung at a funeral or to announce death; figuratively, a sign that something is ending.
From Old English 'cnyllan' meaning 'to strike.' The word's connection to death comes from medieval practice of ringing church bells to announce deaths in the community, making the sound itself symbolic of endings.
John Donne wrote 'never send to know for whom the bell tolls; it tolls for thee'—because in medieval Europe, hearing the knell meant someone in your community had died, so every bell toll was a reminder that death comes for everyone, even you.
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