Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

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

Source : WordNet®

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}, {carry off}]