In a nutshell, at the post office, Bob Ewell "approached [Atticus], cursed him, spat on him, and threatened to kill him." Atticus didn't retaliate; he simply "took out his handkerchief and wiped his face and stood there and let Mr. Ewell call him names." Atticus's reaction was very calm, considering, which just made Bob more angry. So, he asked, "Too proud to fight, you nigger-lovin' bastard?" Atticus responded in his usual classic style by saying, "No, too old," and walked away.
The story spreads through the town like wildfire, and Miss Stephanie Crawford is the main spreader; she is highly amused by Atticus's wry response to Bob, but Scout says of the matter, "Jem and I didn't think it entertaining." They were worried about the threats, and stressed that Bob would act on them. Atticus doesn't seem worried though. His response to the entire thing was, "I wish Bob Ewell wouldn't chew tobacco," referring to the nasty spit that must have ended up on his face.