To Scout in To Kill a Mockingbird, being told she is “like a girl” by her brother Jem is one of the worst insults ever. Scout is a tomboy who despises wearing dresses (much preferring her old overalls) and enjoys playing with Jem and their friend Dill in all the rough-and-tumble ways of boys. So when Jem tells her that she is “like a girl,” it means that she is being prissy, sensitive, and cowardly, and Scout cannot tolerate that. Yet Jem does it anyway, for he knows it will make her angry, and when he is mad at her for something, he finds it the perfect insult.
Jem first tells Scout that she acts “so much like a girl, it's moritfyin’” when she will not go into the Radley yard to get the tire after it accidentally rolls into the yard with Scout inside. She makes Jem go get it, and he is both disgusted with her and unwilling to admit his own fear. A little later, Scout remembers Jem calling her “like a girl” when she doesn’t really want to participate in the pretend-to-be-the-Radleys game, but she does anyway, because she doesn’t want to be seen as just a girl again. The fear of that accusation even keeps Scout away from Jem and Dill’s schemes for a while.
Jem, however, does use the insult again. He and Dill are planning to go up to the Radley place at night, and Scout protests. She is afraid that Boo Radley will kill them. Jem is disgusted yet again at Scout’s hesitancy and declares, “Scout, I’m telling you for the last time, shut your trap or go home ... you’re gettin’ more like a girl every day!” Of course, after that, Scout has to go along, and she gets quite a scare (as do Jem and Dill).