What made Jess angry when his sister called Leslie a stick in Bridge to Terabithia?
The first thing that made Jess so angry was the sheer meanness of his sister's comment. Brenda, "screw(ing) her face up in that ugly way she had," had asked Jess what he was giving his "girl friend." Ellie had interjected that Jess had no "girl friend," and Brenda had agreed with Ellie, commenting lewdly that "nobody with any sense would call that stick a girl." Brenda had "pushed her face right into (Jess's) and grinned the word 'girl' through her big painted lips." The suggestiveness with which Brenda speaks about Leslie contrasts so much with Leslie's innocent genuineness that her insinuations are almost obscene. Fury rises up within Jess, and "if he hadn't jumped out of the chair and walked away, he would have smacked her."
After he has cooled down a little, Jess tries to figure out what had really made him so angry at Brenda's comments. He is used to his sisters' mean pettiness and can usually just ignore them, but this time, he had barely been able to contain himself. After some reflection, Jess realizes that, in addition to the incredible ugliness of Brenda's insinuations, he is angry because her original question about what he is going to get Leslie for Christmas is one that is really bothering him. Even though he knows that Leslie will not expect anything, he wants desperately to get her something meaningful, something that will express how much she means to him in his life. He has been "paralyzed in his efforts to make anything for Leslie," and has no money with which to buy her a gift. Though he knows that Leslie would not laugh at him no matter what he gave her, "for his own sake he had to give her something that he could be proud of" (Chapter 6).