scratch (one's) back
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?
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
scratch someone's back
1. Lit. to scratch, usually with the fingers, the parts of someone's back that they cannot reach. Please scratch my back between my shoulder blades.
2. Fig. to do a favor for someone in return for a favor done for you. You scratch my back, and I'll scratch yours. We believe that the mayor has been scratching the treasurer's back.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.