The Collaborative International Dictionary of English v.0.48:

Quietus \Qui*e"tus\, n. [LL. quietus quit, discharged, L., at
   rest, quiet, dead. See Quiet, a., and cf. Quit, a.]
   Final discharge or acquittance, as from debt or obligation;
   that which silences claims; (Fig.) rest; death.
   [1913 Webster]

         When he himself might his quietus make
         With a bare bodkin.                      --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quietus
    n 1: euphemisms for death (based on an analogy between lying in
         a bed and in a tomb); "she was laid to rest beside her
         husband"; "they had to put their family pet to sleep" [syn:
         rest, eternal rest, sleep, eternal sleep,
         quietus]