pay back (Noun) — Financial return or reward (especially returns equal to the initial investment).
pay back (Noun) — The act of taking revenge (harming someone in retaliation for something harmful that they have done) especially in the next life. ex. "payback is mine; I will repay, saith the Lord"
pay back (Verb) — Act or give recompense in recognition of someone's behaviour or actions.
pay back (Verb) — Take vengeance on or get even.