From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]