At the beginning of Chapter 10, Scout and Jem think of Atticus as "feeble." An older man than most of their schoolmates' parents,
Our father didn't do anything.
The children soon found out differently. When Tim Johnson, the mad dog, came ambling down the road, it was Atticus who Sheriff Tate called upon to take the shot that killed the dog. Jem and Scout soon learned that Atticus had been known as "Ol' One Shot" when he was younger--the best marksman in the county. They were amazed that Atticus had never bragged about this talent before, but Miss Maudie lectured them that
"... he's civilized in his heart. Marksmanship's a gift of God, a talent... People in their right minds never take pride in their talents..."
Jem understood this lesson in humility, and when Scout told him she couldn't wait to tell her friends at school, he told her not to "say anything about it." Although Scout was too young to understand, Jem recognized that Atticus' humble and gentle nature was the mark of a true gentleman. It was a trait of which Jem could be proud, and one that he wanted to emulate himself. That's why he said
"Atticus is a gentleman, just like me."