"The Hands. While standing or kneeling, when the hands are unoccupied, they should be joined before the breast; that is, they are extended and joined palm to palm, the fingers pointing upward at an angle of forty-five degrees, the thumbs crossed right over left. When sitting the hands should be extended one on each knee..."
The Ceremonies of the Roman Rite Described by Adrian Fortescue, J.B. O'Connell, Alcuin Reid, p. 45.
That ancient liturgical norm is related to the ordination ritual and the priest's solemn promise of obedience when the bishop takes the priest's folded hands into his and asks: "N. Do you promise to me and my successors reverence and obedience?" "I do." "May God who has begun this good work in you bring it to perfection." "Amen."
The liturgically folded hands of the priest is at least, therefore, a sign of that solemn promise of reverence and obedience of the priest to his bishop and to the Church and to her doctrine, morality, books, and laws in his devotion to Jesus Christ Himself. A pledge of loyalty! A commitment against error and arbitrariness, even in the liturgy!