In Chapter Six of To Kill a Mockingbird, Jem cleverly manipulates Scout by appealing to the thing she likes least about herself: the fact that she is a girl rather than a boy.
Jem, Dill, and Scout have spent the summer obeying Atticus' demand that they stop "tormenting" Boo Radley with their notes and silly games. However, on the last day that Dill is in Maycomb, he and Jem decide to break the rules and go sneak a peek into the Radley's home through a loose shutter. To silence Scout's protests, Jem tells her:
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!
Because she is a tomboy and does not want to be called out for her so-called "feminine" behavior, Scout agrees to go along with the boys. Their quest ends in terror when they hear a shotgun go off and see a strange shadow of a man with a hat.
Jem says, "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!"
Jem and Dill were Scout's only real "summer" friends, and they were both boys, so she needed to keep up. At this point in the story, Scout doesn't think there is much good in being a female. Later in the story, observation and interaction with Calpurnia open her eyes to the fact that there is "some skill" necessary to be a girl.
Jem does the worst thing that a brother can do to his younger, tomboy sister. He suggests that Scout is too much of a girl to handle this type of adventure. Scout, as evidenced by her choice of clothing, isn't fond of the idea of being a "girl." She wants to be like her older brother and her father, both of whom she idolizes. Jem understands this and plays his cards right.