Grand. There's a word I really hate. It's phony. I could puke every time I hear it.
Holden is disgusted by Mr. Spencer's use of the word "grand" because he believes it is a very phony statement. It is not personalized to any one individual, rather it is a generic statement that teachers, headmasters, and all other "refined" members of society can make about each other. In a sense, they are able to make a comment, without saying anything at all. Fundamentally, it is fake small talk. Also, Holden believes Mr. Spencer is only referring to them as "grand" because they come from the "right" social class, not because they are actually genuinely great people.
Holden is looking for meaning in life; he wants a genuine experience. Therefore, this hollow, fake, small-talk is repulsive to him.
He is dislikes the word "Grand" he believes it is a very phony word, and is disgusted by his teacher's use of the word.