Treachery, violation of faith. “Punic faith” is about equal to “Spanish honesty.” The Puni (a corruption of Poeni) 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 Poeni is the word Phoeni (Phoenicians), the Carthaginians being of Phoenician descent.
Our Punic faith Is infamous, and branded to a proverb.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894