Source : Webster's Revised Unabridged Dictionary (1913)

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 stoke of a bell tolled at a funeral or at the death of a
   person; a death signal; a passing bell; hence, figuratively,
   a warning of, or a sound indicating, the passing away of
   anything.

         The dead man's knell Is there scarce asked for who.
                                                  --Shak.

         The curfew tolls the knell of parting day. --Gray.

Knell \Knell\, v. i. [imp. & p. p. {Knelled}; p. pr. & vb. n.
   {Knelling}.] [OE. knellen, knillen, As. cnyllan. See {Knell},
   n.]
   To sound as a knell; especially, to toll at a death or
   funeral; hence, to sound as a warning or evil omen.

         Not worth a blessing nor a bell to knell for thee.
                                                  --Beau. & Fl.

         Yet all that poets sing, and grief hath known, Of hopes
         laid waste, knells in that word, ``alone''. --Ld.
                                                  Lytton.

Knell \Knell\, v. t.
   To summon, as by a knell.

         Each matin bell, the baron saith, Knells us back to a
         world of death.                          --Coleridge.

Source : WordNet®

knell
     n : the sound of a bell rung slowly to announce a death or a
         funeral or the end of something
     v 1: ring as in announcing death
     2: make (bells) ring, often for the purposes of musical
        edification; "Ring the bells"; "My uncle rings every
        Sunday at the local church" [syn: {ring}]