turnabout is fair play
The phrase "turnabout is fair play" means that if someone has treated you badly or unfairly, it is acceptable for you to treat them in the same way. It suggests that justice or vengeance can be achieved by reciprocating the same actions or behavior towards the person who initiated it. Essentially, it implies that it is justifiable to respond with equal measures when faced with unfair treatment.
An idiom is a phrase or expression that usually presents a figurative, non-literal meaning attached to the phrase. Does that sound fun? It can be!
Play Just Saying - the word game that makes learning idioms a good time on your mobile phone or tablet.