E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Petto.
In petto. In secrecy, in reserve (Italian, in the breast). The pope creates cardinals in petto—i.e. in his own mind—and keeps the appointment to himself till he thinks proper to announce it.
1
“Belgium, a department of France in petto—i.e. in the intention of the people.”—The Herald, 1831.
