do someone a good turn

to do someone a favor without having been asked and without expecting a favor in return

Example: I give to charity because, when I had very little money and no job, someone did me a good turn and now I want to repay the favor by doing the same for someone else.

оказать кому-л. добрую, бескорыстную услугу; оказать кому-л. любезность; удружить.