Are you referring to McMurphy's Moby Dick boxer shorts? In the novel, McMurphy wears these boxer shorts, plays cards with a deck of cards that are sexual in nature and, throughout the novel, relishes sharing stories of his sexual exploits. In fact, that is how he came to be incarcerated in the mental ward. He was jailed for statutory rape and managed to get himself committed because he thought life in the mental ward would be better than working on the prison chain gang. He was very much mistaken.
The Moby Dick boxer shorts.............ah............how can I say this without being censored on this website. Think about some slang terms for the male sexual organ. Then think about these boxer shorts. They have nothing to do with the white whale in Melville's novel, Moby Dick. The boxer shorts are one of the many manifestations of McMurphy's sexuality. I hope you get my gist.
Read about the novel here on eNotes.