↑ "When the prayer [Post-Communion] is over, that one of the deacons appointed by the archdeacon looks towards the pontiff to receive a sign from him and then says to the people: Ite missa est. They answer: Deo gratias" (Ordo I, ed. Atchley, London, 1905, p. 144).
Fortescue, A. (1910), Ite Missa Est, The Catholic Encyclopedia, New York: Robert Appleton Company.
Etymology
One of the oldest formulae of the Roman Rite, recorded in Ordo I (6th or 7th century).[1]
The form missa is an adoption of spoken Late Latin for missio "dismissal".