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).

In Jem and Scout's opinion, acting like a girl is a derogatory comment and means that someone is timid, weak, or cowardly. Jem notices that Scout is reluctant to participate in anything that involves Boo Radley and knows that she will take offense to being called a girl. The children view women as passive, submissive, and uninteresting. Scout is not only raised by a man but spends the majority of her leisure time outdoors with Jem and Dill. She has no desire to behave like a Southern belle, wear a dress, or participate in social events. To be called a girl is the ultimate insult to Scout.

On Dill's last night in Maycomb, the boys plan on trespassing into the Radley yard to get a rare look at Boo through his windows. The boys do not include Scout in their plan because they know she will try to stop them. Once the children meet up, Scout becomes suspicious and demands to know their plan. Eventually, Jem and Dill reveal their plan, and Scout begs her brother to not follow through with it.

Jem responds by saying that Scout is "gettin' more like a girl every day!" Essentially, Jem is saying that Scout is being a coward. Scout is obviously offended by Jem's comment and does not want to be viewed as timid or cowardly. After being called a girl, Scout feels like she has no other choice than to follow along with their plan and trespasses into the Radley yard against her will.

In Chapter 6, Jem and Dill plan their nighttime raid on the Radley yard, and Scout opposes their plan out of fear that Boo Radley will track them down and kill them. After the boys explain to Scout that they simply plan on sneaking into the Radley yard and peeking through their windows to get a look at Boo, she attempts to dissuade them. Jem responds by telling Scout,

"Scout, I’m tellin‘ you for the last time, shut your trap or go home—I declare to the Lord you’re gettin’ more like a girl every day!" (Lee, 53).

Jem's comment insults Scout, who is a tomboy and prides herself on hanging out with her older brother and Dill. Jem knows that Scout is embarrassed to be referred to as a "girl," which is synonymous with the word sissy, and so she will stop complaining about their risky plan. After Jem accuses Scout of acting like a girl, Scout mentions that she has no other choice than to join the boys on their nighttime raid. The children then proceed to sneak into the Radley yard and barely escape after Nathan Radley runs out onto his back porch wielding a shotgun.

Jem knows Scout well. He understands that she sees being a girl as a negative thing. Scout looks around and, with the exception of Miss Maudie, sees women as being weak and restricted by society. She doesn't want to wear dresses and go to tea. She wants to play, like the boys do. Jem is clever older brother -- he knows the best way to convince Scout to do what he wants is to accuse her of being a girl. It's a dare.

Jem is noticing that Scout is becoming more and more hesitant in doing things that involves Boo Radley. I think you might be refering to the part in the novel where the kids are in the Radley garden. As soon as Scout pipes up about her fears, Jem says that Scout is acting like a girl. He sees Scout as nagging him into doing the right thing rather supporting him. Also Dill is with the Finch kids during this episode. Dill does not show any fear in being on the Radley property and he is close in Scout's age. The only reason, Jem reasons, that Scout doesn't want to see Boo is the fact she is a girl.