To repay in kind; take revenge. This term presumably once meant that repayment of a loan would be in exactly the same currency as had been lent. The term dates from Roman times (Plautus and Pliny were among the writers who used it) and was being used figuratively by the sixteenth century. “I did but pay him in’s own coin,” wrote George Chapman in his play The Widdowes Teares (1612).