Uncle Jack Finch tells Scout that she is growing out of her pants in Chapter 9 of To Kill a Mockingbird. What does this mean? Why might he say it?
What does it mean for someone to grow out of their pants? Why would you tell someone they're growing out of their pants?
Uncle Jack's suggestion to Scout that she is "growing out of her pants" is merely an expression, similar to "getting too big for your britches": She is too cocky and outspoken. Uncle Jack simply means that Scout is growing up and is learning (too quickly, he believes) many adult words and expressions. Jack makes this comment after Scout had cursed earlier in the day and, then, again at the dinner table, when she requests that he "pass the damn ham, please." He instructs her to have a little talk with him afterward, and Jacks attempts to explain to Scout that using words like "damn and hell" is not ladylike. It doesn't have much of an effect, however. When Jack asks "You want to grow up to be a lady, don't you?", Scout responds "not particularly."
In Harper Lee's To Kill a Mockingbird, when Uncle Jack tells Scout Finch that she is "growing out of her pants," there are several layers to this comment. First of all, there is the parallel to the phrase "getting too big for your britches," as noted by other answers. It indicates that Scout is becoming too stuck up and self important.
Beyond that, there is the context of the entire setting and background of the novel. Throughout the novel, Scout ages from six to eight years old. At this point in her life, she has always been a "tomboy." She has rejected traditional feminine roles and ideas. She is called the gender ambiguous nickname "Scout," rather than her very feminine given name, Jean Louise. She loathes everything ladylike, as demonstrated by her strained relationship with her proper Aunt Alexandra. But for a young girl to do these things (even today, but especially in the American South during the era of the Great Depression), she is considered deviant. Because Scout is very young and has only been raised by a single father, she is often "forgiven" by relatives and community members for her boyishness and rejection of the feminine. However, people around her believe that it is time for her to start learning how to be more of a lady. (A large part of the reason why Aunt Alexandra comes to visit in the first place is to "help" Scout become a lady.) Uncle Jack's comment basically implies, "Scout, you are growing up, which means that it is becoming less and less okay for you to act like a boy. You have to start conforming to societal norms and presenting yourself publicly as a lady." The "pants" seem to be metaphoric. They represent the boyish lifestyle that Scout adores and yearns to keep. She struggles against the "dress" lifestyle of ladyhood. Yet, she is beginning to realize that people around her are expecting her to act in a way she does not want to act. She does not want to "grow out of her pants," even as the society she lives in continually insists that she should.
I interpret this as Uncle Jack's way of telling Scout that she is getting to an age where she can behave as a lady and that, as a lady, she should not be cussing.