Idioms and Phrases with scratch someone's back
scratch someone's back
Do someone a favor in hopes that a favor will be returned. For example, I don't mind driving this time—she's scratched my back plenty of times. It also is put as you scratch my back and I'll scratch yours, as in If you do the laundry I'll do the cooking—you scratch my back and I'll scratch yours. This idiom was first recorded in 1704.