To provide one with a favor; to do something generous, helpful, or beneficial for someone. Usually implies that the favor is expected or hoped to be returned in the future. The president has been accused of scratching the oil industry's backs with a series of executive actions that put them in a position to make a lot more money. I've been scratching his back for years now—getting him a job, helping him move into his apartment, lending him money to buy furniture. The least he could do is help me clean out my house before I move. Just get me the information on the senator I need, and I'll make sure it's worth your while—you scratch my back and I'll scratch yours, OK?