pay back

1.  Repay a debt or a loan, as in I'll pay you back next month.
2.  Also, pay back in someone's own coin. Revenge oneself, repay in kind, as in He thought he could get away with copying my plans, but I'll pay him back in his own coin. This expression refers to repaying a debt in exactly the same currency in which the money had been lent. [c. 1600]

The verb has 2 meanings:

Meaning #1: act or give something in recognition of someone's behavior
  Synonyms: reward, repay

Meaning #2: take vengeance on or get even
  Synonyms: pay off, get, fix