[1] : WordNet (r) 2.0

pay back
     v 1: act or give recompensation in recognition of someone's
          behavior or actions syn: reward, repay
     2: take vengeance on or get even; "we'll get them"; "that'll
        fix him good"; "this time i got him" syn: pay off, get,
         fix
see also:
reward repay pay off get fix 

[2] : Moby Thesaurus II by Grady Ward, 1.0

58 moby thesaurus words for "pay back":
   alternate, atone, bandy, be quits with, change, commute,
   compensate, cooperate, counterchange, cover, do penance, exchange,
   expiate, fill up, get back at, get even with, get satisfaction,
   give and take, give satisfaction, indemnify, interchange,
   kick back, logroll, make amends, make compensation, make good,
   make reparation, make requital, make restitution, make retribution,
   make up for, make up to, pay, pay damages, pay in kind, pay off,
   permute, quit, reciprocate, recompense, recoup, rectify, redress,
   refund, reimburse, repay, requite, respond, retaliate, return,
   return the compliment, reward, set right, square, swap, switch,
   trade, transpose

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