"Tell me what she said to you, Owen," the headmaster said.
"IT WAS VERY UGLY," said Owen Meany, who actually thought he was protecting the president of the United States! Owen Meany was protecting the reputation of his commander-in-chief!
"Tell him, Owen!" I said.
"IT IS CONFIDENTIAL INFORMATION," Owen said. "YOU'LL JUST HAVE TO BELIEVE ME—SHE WAS UGLY. SHE DESERVED A JOKE—AT HER OWN EXPENSE," Owen said. (7.373-376)
The headmaster wants to know why Owen made an inappropriate sexual remark about Mrs. Lish. We know that it's because she was making speculations about President Kennedy's rumored affair with Marilyn Monroe, and these speculations offended Owen. Still, even though he could potentially get in a whole heap of trouble, Owen refuses to tell the backstory; he firmly believes that he is protecting the President's reputation.