The Collaborative International Dictionary of English v.0.48:

Decimate \Dec"i*mate\, v. t. [imp. & p. p. Decimated; p. pr. &
   vb. n. Decimating.] [L. decimatus, p. p. of decimare to
   decimate (in senses 1 & 2), fr. decimus tenth. See
   Decimal.]
   1. To take the tenth part of; to tithe. --Johnson.
      [1913 Webster]

   2. To select by lot and punish with death every tenth man of;
      as, to decimate a regiment as a punishment for mutiny.
      --Macaulay.
      [1913 Webster]

   3. To destroy a considerable part of; as, to decimate an army
      in battle; to decimate a people by disease.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decimate
    v 1: kill one in every ten, as of mutineers in Roman armies
    2: kill in large numbers; "the plague wiped out an entire
       population" [syn: eliminate, annihilate, extinguish,
       eradicate, wipe out, decimate, carry off]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "decimate":
   annihilate, bring to ruin, butcher, commit carnage,
   commit genocide, commit mass murder, condemn, confound, consume,
   damn, deal destruction, demolish, depopulate, depredate, desolate,
   despoil, destroy, devastate, devour, dissolve, engorge,
   exterminate, gobble, gobble up, gut, gut with fire, havoc,
   incinerate, lay in ruins, lay waste, massacre, murder wholesale,
   ravage, raze, ruin, ruinate, shipwreck, slaughter, slay en masse,
   swallow up, throw into disorder, unbuild, undo,
   unleash destruction, unleash the hurricane, unmake, upheave,
   vandalize, vaporize, waste, wipe out, wrack, wreak havoc, wreck