E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Twopenny Dam (A).
A dám was an Indian coin and weight. Originally a gold mohur contained sixteen dáms; a punchee was = the quarter of a dam, and a bárahgáni = half a punchee. Putting this in a tabular form, it would be thus:
2 bárahgánis = 1 punchee.
4 punchees = 1 dam (worth in its diminished value about 2 pence).