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Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs