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]

Moby Thesaurus II by Grady Ward, 1.0:

178 Moby Thesaurus words for "quietus":
   KO, Waterloo, Z, absolution, acquittal, acquittance, annihilation,
   apodosis, bane, beating, biological death, catastrophe, ceasing,
   cessation, cessation of life, clearance, clearing, clincher,
   clinical death, coda, collapse, compurgation, conclusion,
   conquering, conquest, consummation, crack of doom, crash,
   crossing the bar, crusher, culmination, curtain, curtains, death,
   death knell, death stroke, deathblow, debacle, debt of nature,
   decease, defeat, demise, denouement, departure, destigmatization,
   destigmatizing, destination, destiny, destruction, discharge,
   disculpation, dismissal, dissolution, doom, downfall, drubbing,
   dying, ebb of life, effect, end, end of life, end point, end-all,
   ender, ending, envoi, epilogue, eschatology, eternal rest,
   exculpation, excuse, exit, exoneration, expiration, extinction,
   extinguishment, failure, fall, fate, final solution, final stroke,
   final summons, final twitch, final words, finale, finality,
   finger of death, finis, finish, finisher, finishing stroke,
   forgiveness, goal, going, going off, grave, hand of death, hiding,
   izzard, jaws of death, kayo, kayo punch, knell, knockout,
   knockout blow, lambasting, last, last breath, last dab, last debt,
   last gasp, last muster, last rest, last roundup, last sleep,
   last things, last trumpet, last words, lathering, latter end,
   leaving life, licking, loss of life, making an end, mastery, omega,
   overcoming, overthrow, overturn, pardon, parting, passing,
   passing away, passing over, payoff, period, perishing, peroration,
   purgation, purging, quittance, release, remission, resolution,
   rest, resting place, reward, ruin, sentence of death, settler,
   shades of death, shadow of death, silence, sleep, smash,
   sockdolager, somatic death, stoppage, stopper, stopping place,
   subdual, subduing, subjugation, summons of death, swan song, term,
   terminal, termination, terminus, thrashing, trimming, trouncing,
   undoing, vanquishment, verdict of acquittal, vindication, whipping,
   windup



Bouvier's Law Dictionary, Revised 6th Ed (1856):

QUIETUS, Eng. law. A discharge; an acquittance.
     2. It is an instrument by the clerk of the pipe, and auditors in the
exchequer, as proof of their acquittance or discharge to accountants. Cow.
Int. h.t.