I believe the answer you're looking for is
axis; it
I believe the correct answer is naive.
The author is trying to say that he was ignorant, and that he couldn't understand properly these new revelations. He was either too young or too stupid to understand such a thing, but he believes he will as he grows older.
It's The lightning thief, The sea of monsters, The titan's curse, The battle of Labyrinth, and Percy Jackson's Greek Gods. (That is the sequence of the book series.)
