'To pay one in his own coin' definitions:

Definition of 'To pay one in his own coin'

From: GCIDE

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. [1913 Webster]
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. [1913 Webster]
It is alleged that it [a subsidy] exceeded all the current coin of the realm. --Hallam. [1913 Webster]
3. That which serves for payment or recompense. [1913 Webster]
The loss of present advantage to flesh and blood is repaid in a nobler coin. --Hammond. [1913 Webster]
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.] [1913 Webster]