Jem's definition of a girl changes as he matures throughout the course of Harper Lee's To Kill a Mockingbird. We learn Jem's definitions of a girl through the various accusations he occasionally hurls at Scout.
The first time we see Jem accuse Scout of acting like a girl is when the children are playing with the spare tire. Dill has just returned for his second summer in Maycomb, and Scout has angered Jem by calling his myth concerning Hot Steams "nigger-talk" and telling Dill not to believe any of it. When Scout suggests they play by rolling in the tire, Jem unleashes his anger by pushing her, balled up in the tire, with all his might towards Radley Place. The tire crashes into the porch of Radley Place and dumps Scout onto the pavement. She is so dizzy that at first she can't move. When she realizes where she is, she stands, "trembling," and runs off of the property, abandoning the tire. Jem yells at her to get the tire, but she refuses, commanding him to go and get it instead. When Jem finally acquiesces and returns with the tire, he says to her, "See there? ... Nothin' to it. I swear, Scout, sometimes you act so much like a girl it's mortifying" (Ch. 4). Based on the context of his comment, we can tell that Jem is equating being a girl with showing a lack of courage. According to Jem's young mind, only boys are brave; therefore, Scout acts like a girl anytime she shows fear.
During this second summer with Dill, Jem continues to accuse Scout of behaving like a girl anytime she hesitates to play the "Boo Radley game" Jem has invented in which the children act out the rumors and myths surrounding Arthur Radley's life, their neighbor who the children call Boo Radley.
However, Jem's encounter with Mrs. Dubose and his father's lesson concerning Mrs. Dubose changes him significantly, helping him to see many things through a new, mature perspective. On the surface, Mrs. Dubose is the meanest old woman in the neighborhood. However, upon her death, Atticus tells Jem that she was really a "great lady" and the "bravest person [Atticus] ever knew" (Ch. 11). Atticus sees her as a great and brave lady because she undertook ridding herself of her morphine addiction despite the amount of pain she was in due to her illness and despite her imminent death. As Atticus explains to his children, real courage is knowing "you're licked before you begin but you begin anyway and you see it through no matter what" (Ch. 11). Once Jem is able to see Mrs. Dubose as a great and brave lady, his perception of what it means to be a girl completely changes. In Chapter 12, Scout begins complaining because Jem has been acting differently since Mrs. Dubose's death. At one point, Jem yells at Scout, saying, "It's time you started bein' a girl and acting right!" and making her cry. While we don't know what Scout did to provoke Jem's outburst, we do know that his perception has been changed by his experience with Mrs. Dubose. Evidently, Jem is now able to see a girl, or true lady, as one who behaves with the utmost bravery, just as Mrs. Dubose did in the final days of her life.
Hence, as Jem matures, his definition of being a girl changes from being one who isn't brave enough to being one who is extremely courageous.