To pay back money that was borrowed or owed. To return a favor, kindness, or injury in kind.
From Old French 'repayer,' composed of 're-' (back, again) and 'payer' (to pay), from Latin 'pacare' meaning 'to pacify' or 'satisfy.' The word evolved from literal monetary payment to include metaphorical repayment of favors, kindnesses, or wrongs by the 14th century.
The concept of repayment is so fundamental to human society that anthropologists consider it one of the basic social contracts - the obligation to reciprocate creates and maintains relationships across cultures. Interestingly, studies show that people often feel more obligated to repay small unexpected favors than large expected ones, a principle marketers exploit constantly.
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