The Collaborative International Dictionary of English v.0.48:

Coin \Coin\ (koin), n. [F. coin, formerly also coing, wedge,
   stamp, corner, fr. L. cuneus wedge; prob. akin to E. cone,
   hone. See Hone, n., and cf. Coigne, Quoin,
   Cuneiform.]
   1. A quoin; a corner or external angle; a wedge. See
      Coigne, and Quoin.
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   2. A piece of metal on which certain characters are stamped
      by government authority, making it legally current as
      money; -- much used in a collective sense.
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            It is alleged that it [a subsidy] exceeded all the
            current coin of the realm.            --Hallam.
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   3. That which serves for payment or recompense.
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            The loss of present advantage to flesh and blood is
            repaid in a nobler coin.              --Hammond.
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   Coin balance. See Illust. of Balance.

   To pay one in his own coin, to return to one the same kind
      of injury or ill treatment as has been received from him.
      [Colloq.]
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