1.
If you pay back some money that you have borrowed or taken from someone, you give them an equal sum of money at a later time.
He burst into tears, begging her to forgive him and swearing to pay back everything he had stolen...
I’ll pay you back that two quid tomorrow.
PHRASAL VERB : V P n (not pron) , V n P n , also V n P
2.
If you pay someone back for doing something unpleasant to you, you take your revenge on them or make them suffer for what they did.
Some day I’ll pay you back for this!
PHRASAL VERB : V n P for n , also V n P