do someone a favour

do (someone or oneself) a favor

1. To help someone else, typically at their request. In this usage, the person being helped is stated between "do" and "a." Hey, do me a favor and take these bags into the kitchen while I get the others from the car.
2. To do something to help or better oneself. In this usage, a reflexive pronoun is used between "do" and "a." Do yourself a favor and go to college—now that I'm older, I regret not having that experience. Because I knew that I had to get up at 4 AM today, I did myself a favor and went to bed early last night.
3. A request for someone to stop doing something bothersome or annoying. Can you please do me a favour and take your loud music somewhere else? I'm trying to sleep!
4. A response to a statement that the speaker finds ridiculous or stupid. Primarily heard in UK, Australia. A: "Katie seems nice." B: "Oh, do me a favour—she's one of the cattiest girls in school!"
See also: favor

do someone a favour

do something for someone as an act of kindness. British informal
See also: favour, someone