As Chapter 6 begins, it's almost Christmas, and Jess's sister Brenda really makes Jess angry by calling his best friend Leslie a stick (meaning she's skinny and tall, and not busty or curvy). Let's take a look at the context:
"What are you giving your girl friend, Jess?" Brenda screwed her face up in that ugly way she had. He tried to ignore her. He was reading one of Leslie's books, and the adventures of an assistant pig keeper were far more important to him than Brenda's sauce.
"Don't you know, Brenda?" Ellie joined in. "Jess ain't got no girl friend."
"Well, you're right for once. Nobody with any sense would call that stick a girl." Brenda pushed her face right into his and grinned the word "girl" through her big painted lips. Something huge and hot swelled right up inside of him, and if he hadn't jumped out of the chair and walked away, he would have smacked her.
So why does this make Jess so angry? He thinks about it himself, surprised even at his own anger. Here's what he comes up with:
1. He thinks his sister Brenda is dumb, that Leslie is brilliant and wonderful, and he's mad that Brenda even believes she's allowed to criticize Leslie.
2. Brenda's behavior is so annoying that it makes Jess consider briefly if he really is her flesh-and-blood relative or if somehow he joined the family as a little foundling. Jess is probably reeling at the idea that Brenda's words could have this effect on him, making him question his sense of belonging in the family.
3. The whole situation made him realize that he really doesn't have anything to give Leslie for Christmas, and he has no way of making money to buy her something. It's frustrating, because he wants to express his friendship to Leslie with a gift that will mean something to her. Brenda's comment just made him realize how inadequate he will be when he does try to find a present for Leslie.