Borrowed from Italian, the adverb in petto means in the secret of the mind or heart.

The Italian noun petto, denoting the breast, is from classical Latin pectŭs/pectŏr-, denoting the breast in its literal sense and as the seat of emotions, of intelligence, of thinking—cf. also of that kidney.

The adverb in petto first appeared in English in The Moderate Intelligencer: Impartially communicating Martial Affaires to the Kingdome of England (London) of 4^th November 1647, as a term specific to the Roman Catholic Church:

There being no more Places vacant in the sacred Colledge, after that the seventh (reserved in petto) hath been nominated by his Holyship.

Auguste Boudinhon (1858-1941), professor of canon law, explained this specific use in the eighth volume of The Catholic Encyclopedia (New York, 1910):

In Petto, an Italian translation of the Latin ‘in pectore’, “in the breast”, i.e. in the secret of the heart. It happens, at times, that the pope, after creating some cardinals in consistory, adds that he has appointed one or more additional cardinals, whom he reserves ‘in petto’, and whom he will make known later: “alios autem [v.g. duos] in pectore reservamus, arbitrio nostro quandoque declarandos.” Until they have been publicly announced these cardinals acquire no rights, and if the pope dies before having declared their names they do not become members of the Sacred College; but when he has proclaimed their elevation at a subsequent consistory, they take rank from the date of their first nomination and receive from that date all the emoluments accruing to their office. This is a method that the popes have sometimes adopted to ensure poor ecclesiastics a competency to meet all the expenses incident to their promotion. At the consistory of 15 March, 1875, Pius IX announced that he was creating and reserving ‘in petto’ five cardinals, whose names would be found, in case of his death, in a letter annexed to his will. But the canonists having raised serious doubts as to the validity of such a posthumous publication, Pius IX published their names in the consistory of the following 17 September.

The first known user of in petto without relation to the Roman Catholic Church was the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745) in A Discourse of the Contests and Dissensions between the Nobles and the Commons in Athens and Rome, with the Consequences they had upon both those States (London, 1701):

In order to preserve the Balance in a mix’d State, the Limits of Power deposited with each Party ought to be ascertained, and generally known. The defect of this is the cause that introduces those strugglings in a State about Prerogative and Liberty, about Encroachments of the Few, upon the Privileges of the Many, and of the Many upon the Rights of the Few, which ever did and ever will conclude in a Tyranny; First, either of the Few, or the Many, but at last infallibly of a single Person. For, which ever of the three Divisions in a State is upon the Scramble for more Power than its own (as one or other of them generally is) unless due care be taken by the other two; upon every new Question that arises, they will be sure to decide in favour of themselves, talk much of Inherent Right; they will nourish up a dormant Power, and reserve Privileges in petto, to exert upon Occasions, to serve Expedients, and to urge upon Necessities. They will make large Demands, and scanty Concessions, ever coming off considerable Gainers: Thus at length the Balance is broke, and Tyranny let in, from which Door of the three it matters not.

In English, apparently by confusion with the adjectives petit, petty, the adverb in petto has come to also mean in miniature, on a small scale. On 30^th November 1901, Notes and Queries (London) published the following letter from a correspondent signing themself ‘H’:

“In Petto.”—May I ask whether this Italian phrase can under any circumstances properly convey the meaning of “in miniature”? I had always understood that “avere qual cosa in petto” meant having a certain matter in one’s mind, with the sense of its being kept secret, but I lately saw a quotation from ‘A Journey to Nature’ (Constable & Co.) in one of our literary weekly papers, in which it evidently bore the sense of “in miniature,” without comment from the reviewer. The passage runs as follows:—
“One hot day we lay flat on our stomachs under the shade of a beech, among the June grass and the daisies, peering down into a magic spectacle, and yet it was the planet’s history in petto. The great loom of the universe was working there with miniature continents,” &c.
This use of in petto appears to be such a case of Italianized French as is described so amusingly by Mr. Carmichael in ‘In Tuscany,’* and the author of ‘A Journey to Nature’ has evidently jumped to the conclusion that in petto is equivalent to en petit. As Mr. Carmichael says:—
“One of the first happy thoughts of the beginner is to Italianize French words. It answers so often. He knows, to begin with, that if he changes the French eau into ello (e.g., agneau, agnello) or the French eur into ore (e g., vapeur, vapore) he will probably be right. He is tempted to soar beyond these ascertained rules, garçon, garzone; jardin, giardino; hier, ieri; jamais, giammai; how smoothly the system works. He goes into a pizzicheria and asks the price of jambon, giambone, pointing to a small juicy ham of the Casentino cure. ‘Questo giambone,’ says the courteous shopman, ‘costa novanta centesimi la libbra.’ The ham is bought on the spot, and sent home: the cook is asked what she thinks of the giambone. ‘The what!’ she asks, in bewildered astonishment. ‘The giambone which I myself sent home from the pizzicheria.’ ‘Ah!’ she gasps apologetically, ‘it is excellent giambone. Will the signore have some of it fried with eggs after the manner of the Americans?’
“And so, thanks to an infamous conspiracy of courtesy between a shopman, a cook, a parlour-maid, and a serving-man, it was six months before I found out that there was no such word in the Tuscan tongue as giambone, and that the Italian for ham was prosciutto!”

* In Tuscany: Tuscan towns, Tuscan types and the Tuscan tongue (New York, 1901), by Montgomery Carmichael (1857-1936), English diplomat and author