It 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.
“Belgium, a department of France in petto —i.e. in the intention of the people.” —The Herald, 1837.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894