E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Treachery, violation of faith. Punic faith is about equal to Spanish honesty. The Puni (a corruption of Pni) were accused by the Romans of breaking faith with them, a most extraordinary instance of the pot calling the kettle black; for whatever infidelity the Carthaginians were guilty of, it could scarcely equal that of their accusers.
The Roman Pni is the word Phni (Phnicians), the Carthaginians being of Phnician descent.