Didn't want to spoil anything in the title, but the party where Quentin was planning on wearing a blazer with a grass skirt and it ends with him sleeping with Janet??
The word basically meant the pairing of two normally incompatible things. I don't have the books with me at the moment and no amount of searching will give me the word!
Side note: the moment in the party where it all suddenly becomes perfect and happy was one of my favorite parts of the series, just like their nights in the cottage. As someone who values close friendships but often feels alienated, those moments are the happiest moments in my life, and it feels wonderful to read Grossman's description of them.
