Afraid of getting hurt.

"I never fight duels," said Mr. Rowland firmly.
"Why not?" demanded the other unpleasantly.
"I'm too afraid of getting hurt."
"Aha! Is that so? Then I will at least pull your nose for you."
The young man advanced fiercely. Exactly what happened was difficult to see, but he described a sudden semicircle in the air and fell to the ground with a heavy thud. He picked himself up in a dazed manner. Mr. Rowland was smiling pleasantly.
"As I was saying," he remarked, "I'm always afraid of getting hurt. That's why I thought it well to learn jujitsu."

-- Agatha Christie, "The Girl in the Train"

It works beautifully in novels :)