As the Church of Rome grew stronger, it acquired converts in many of the lands both near to and distant from Italy, including those in Europe to the north.
And, of course, to retain these converts, it was necessary to appoint representatives in these countries who would represent the Church to the natives.
Now, the north of Italy is bounded by mountains (the Alps), and it became the practice to refer to one of these representatives as ultramontanus, “one who is beyond the mountains,” which is made up of the Latin ultra, “beyond,” plus mont-, the combining form of mons, “mountain,” together with a masculine personal suffix.
The word (with suitable variations appropriate to the language) was taken into French as early as the fourteenth century, and also into Spanish, Portuguese, Italian, German, and Dutch.
It entered English in the late sixteenth century, since which time there have been a number of minor variations in meaning.
That now most commonly in use is “one who supports papal policy.”