The act of scratching one's back (or someone else scratching your back), also used figuratively to mean doing favors for someone in hopes they'll return the favor.
Present participle of 'backscratch'. The figurative meaning (mutual favor-trading) developed because the literal action requires cooperation from another person.
The figurative meaning 'you scratch my back, I'll scratch yours' became so universal that it appears in writings from ancient Rome, medieval China, and modern politics—it seems like humans everywhere recognized that interdependence creates mutual obligation.
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