How did Tolkien learn the word wizard

Why do people talk about wizards as if they were common in The Lord of the Rings?

This is largely covered in "The Unfinished Tales"

Basically there were (initially) only 5 of the Istari.

Wizard is a translation of Quenya istar (Sindarin ithron): one of the members of an 'order' (as they called it) which claims to possess and exhibit outstanding knowledge of the history and nature of the world.The translation (although appropriate in relation to "wise" and other ancient knowledge words, similar to that of istar in Quenya) may not be happy as the Heren Istarion or the "Order of Sorcerers" was markedly different from the "Wizard". and "magician" of later legend;They belonged exclusively to the Third Age and then left, and no one but Elrond, CĂ­rdan, and Galadriel discovered what kind they were or where they came from.

You spent a lot of time with important and powerful people

Among men, they were (at first) accepted by those who had anything to do with them, as men who had acquired traditions and arts through long and secret study.They first appeared in Middle-earth around the year 1000 of the third age, but for a long time they wandered around in simple form, from men who were years old, so to speak, but healthy in body, from travelers and hikers who had knowledge over Middle-earth and everything that dwelt in it, but did not reveal their powers and intentions to anyone.During this time, people rarely saw them and paid little attention to them.

As Sauron's power grew, people paid them a hell of a lot more attention

But as the shadow of Sauron began to grow and take shape again, they became more active, always trying to contest the shadow's growth and to persuade elves and humans to guard against their danger.Then a rumor spread far and wide among the people of their comings and goings and their interference in many affairs;

At this point it was noticed that they are not aging.This gave them celebrity status as they now had hundreds of years to get known

and people perceived that they did not die but remained the same (unless they aged a little in appearance) while the fathers and sons of men died.As a result, humans began to fear them more and more, even if they loved them, and they were viewed as belonging to the elves (with whom they had, in fact, often hooked up).