One Good Turn Deserves Another: Origin and Meaning
This idiom has been in English for a long time- since at least the 1650s, though probably in spoken English for longer. It is thought of as a translation of the older Latin Quid pro quo- something for something else, which is sometimes used in more formal English with the same meaning as our idiom here.
– Well, she helped me with my maths, and one good turn deserves another!
– Come on, help us out here. We didn’t tell the police about your stealing, and one good turn deserves another.