The Collaborative International Dictionary of English v.0.48:

Knell \Knell\, n. [OE. knel, cnul, AS. cnyll, fr. cnyllan to
   sound a bell; cf. D. & G. knallen to clap, crack, G. & Sw.
   knall a clap, crack, loud sound, Dan. knalde to clap, crack.
   Cf. Knoll, n. & v.]
   The stroke of a bell tolled at a funeral or at the death of a
   person; a death signal; a passing bell; hence,
   (figuratively), a warning or harbinger of, or a sound
   indicating, the passing away of anything; -- also called
   death knell.
   [1913 Webster +PJC]

         The dead man's knell
         Is there scarce asked for who.           --Shak.
   [1913 Webster]

         The curfew tolls the knell of parting day. --Gray.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

death knell \death" knell`\, n.
   1. A stroke or tolling of a bell, announcing a death; a
      knell[1].
      [1913 Webster]

   2. Hence: (figuratively) A sign or harbinger of the end,
      death, or passing away of anything.
      [PJC]

WordNet (r) 3.0 (2006):

death knell
    n 1: an omen of death or destruction
    2: a bell rung to announce a death [syn: death knell, death
       bell]