The term ‘ordinary magisterium’ was first used by Pius IX in the letter Tuas libenter addressed to the archbishop of Munich and Freising on 21 December 1863.
Earlier that year, a meeting of Catholic theologians had taken place in Munich. The pope had been told that in the course of that meeting the opinion had been expressed that Catholic theologians were bound to hold only those truths of faith which had been solemnly declared.
Pius IX replied that “it must not be limited to those things which have been defined by the express decrees of councils or of the Roman Pontiffs and of this Apostolic See, but must also be extended to those things which are handed on by the ordinary magisterium of the whole church dispersed throughout the world as divinely revealed, and therefore are held by the universal and constant consensus of Catholic theologians to pertain to the faith.”
