When we think about numbers, we don’t often consider that once upon a time, they didn’t go very high. But in fact, in ancient days, there weren’t words for anything greater than “ten thousand.” In the Ancient Greek and Roman eras, this was the largest number known, and myriad was the world used for it.
It was in fact because this was the largest named number that myriad also came to mean “countless, innumerable, vast amount.”
So what about million? It literally means “great thousand” and didn’t come along in any language until the 13th century. Even once it had taken on a more precise meaning, it was used only by mathematicians up into the 16th century!