In chapter 7 of Their Eyes Were Watching God, Janie has been married to Jody Starks for a number of years. By this point, she is fed up with the monotony of working in the shop, and Jody continues to belittle her every chance he gets. The years have left Janie weary, and she frequently fantasizes about running away from this life. However, the best that she can resign herself to is to wait for her husband to die.
At one point, Jody makes fun of Janie's looks and calls her old in front of a store full of customers. Pushed to her breaking point, Janie snaps. With the whole store porch full of customers as her audience, she points out that Jody is even older than her and that age has made him useless as a lover. This gets quite the reaction from the other men at the store.
This is particularly insulting to Jody. First of all, he is publicly humiliated now that the whole town knows that he cannot control his wife. His manhood is also very important to him. To be emasculated in front of his social circle like this is too much for his bravado to bear. He responds to his wife's words with violence, striking her as hard as he can.