Latin professor meant someone who publicly claims expertise, not someone who teaches.
The English word "professor" appeared in the late fourteenth century from Old French professeur and directly from Latin professor, meaning a person who professes to be an expert in some art or science, or a teacher of the highest rank.1 The root was the same as profession: Latin profitēri, to declare openly. A professor was, at the level of language, someone who stood before others and said, publicly, that they knew something.
The word carried weight because the declaration was public. In medieval universities, the title indicated that a scholar had been examined and found qualified to teach. Teaching was the purpose; research was not yet part of the role.2
The meaning of the title shifted in the nineteenth century when the German university system, particularly after the founding of the University of Berlin in 1810, redefined the professor as both teacher and researcher. Wilhelm von Humboldt's model insisted that the two activities were inseparable: a professor who did not produce original knowledge had no business teaching.3
Until the early twentieth century, few academic staff in English-speaking universities held doctorates. The German practice of requiring lecturers to hold research doctorates gradually spread. Today, a PhD is a near-universal prerequisite for a professorial appointment in most countries.4
By the 1700s, "professor" was being used as a formal title prefixed to a name, a practice that persists across the English-speaking world. The informal shortening "prof" appeared by the mid-nineteenth century. In some countries, "professor" remains strictly limited to holders of named or senior academic chairs. In others, particularly the United States, it applies to any faculty member who teaches at a university.5
The distance between the original meaning and the modern one is worth noting. A medieval professor declared what was known. A modern professor is expected to discover what is not yet known and then declare it. The word stayed the same. The job description inverted.6