What does the prefix 'meta' REALLY mean?

"After" That's what it meant in ancient Greek. The Word "metaphysics" got its name because in the list of Aristotle's books, the Metaphysics was the book that came right after the Physics. However, the Word became enormously popular as the philosophy of Aristotle became popular in the Middle Ages and people started to imagine that the prefix "meta" had to do with the relation of the content of the Metaphysics, as opposed to that of the Physics and people started to use it to mean the overarching or underlying principles behind something (as the Metaphysics dealt with the underlying principles of Physics). Thus in newly-formed Words like metacognition, the prefix is used to mean "underlying principles of".