The word magnus means "great" in Latin, being where we get the terms "magnification", "magnitude", and similar words relating to large size or figurative greatness. For example, Pompey the Great was called Gnaeus Pompeius Magnus. However, in fantasyland, it seems to mean "great mage." Most of the times you see a character named Magnus in a fantasy work, he is a mage, wizard or something like that. It probably doesn't help that magus, the actual Latin word for "mage," is just one