What is the story behind the dress in the tree?

On the ride back to the institution, Candy falls asleep against Billy's chest. He later asks her for a date. McMurphy plans to sneak Candy into the ward on Saturday night so she can meet with Billy. McMurphy seems exhausted on the trip back to the institution. He points out the house where he lived as a youth and points out a dress hanging in the branches of a tree. The first girl who dragged McMurphy to bed wore that dress, and it now stands in the tree as a de facto memorial. He was about ten at the time.