Holden thinks that anyone who pretends to be someone he is not is a phony.
The first example of a phony Holden gives is the headmaster at his prep school. He considers him the ultimate phony. He gives several examples of him putting on airs, including only talking to
Grand. There's a word I really hate. It's a phony. I could puke every time I hear it. (ch 1)
Holden also has a problem with girls who pretend to like him, or who he thinks pretend to like him.
I spent the whole night necking with a terrible phony named Anne Louise Sherman. Sex is something I just don't understand. (ch 9)
To Holden, most interactions with people are phony. He feels that there are very few genuine people in the world. Most adults, in particular, are phony. People pretend to like you, and pretend to care, but are just phony. It is one of the reasons he feels so alone.