In Chapter 3, Ackley comes into the room Holden shares with Stradlater. Holden is trying to read Out of Africa by Isak Dinesen and deliberately ignores Ackley, hoping he will go away. 

He started walking around the room, very slow and all, the way he always did, picking up your personal stuff off your desk and chiffonier. He always could get on your nerves sometimes.

This is one of the things that bother Holden. Ackley "gets on his nerves" and also revolts him with his bad manners. As a specific example of the "personal stuff" Ackley picks up Stradlater's knee supporter. 

"Who belongs this?" Ackley said.
I told him it was Stradlater's. So he chucked it on Stradlater's bed. He got it off Stradlater's chiffonier, so he chucked it on the bed.

Ackley not only bothers Holden with his bad manners, but he disgusts him with his unclean personal habits. After borrowing Holden's scissors, Ackley starts cutting his "big, horny-looking" fingernails. Holden protests:

"Cut 'em over the table, willya? I don't feel like walking on your crumby nails in my bare feet tonight." He kept right on cutting them over the floor, though. What lousy manners. I mean it.

Besides characterizing Ackley as a boorish slob, the incident creates an impression of dorm life in general for young males in boarding schools, fraternities, military barracks, and elsewhere. There are always intrusions, bad manners, insults, and other such things that bother sensitive people like Holden but do not seem to bother insensitive people like Ackley and Stradlater. Holden is an introvert, a loner. He leads a solitary existence. The fact that he is reading a book about the African wilderness suggests that he is an escapist. Even in Manhattan, he seems to be all alone among those teeming millions. It is very much "in character" for Holden to be trying to read a classic book and "in character" for Ackley to be both consciously and unconsciously antagonizing and disgusting him with his uncouth manners and his annoying personal questions.