Vox.com's Ezra Klein asked him about that in the interview below at the 30:30 mark.
Klein: So I have a couple very specific questions about your book. I would like to hear more about the time you called Dinesh D'Souza a dick.D'Souza is under federal criminal indictment for allegedly violating campaign finance law.
Geithner: There was a group of conservative students at that point that started a conservative newspaper called the Dartmouth Review and they published in their newspaper a confidential list of members of the Lesbian-Gay Student Alliance, including a bunch of people who hadn't come out to their friends or their parents. And it was devastating to them. They did it without their consent. And so I ran into him [D'Souza] in a line at the dining hall or one of the cafes and said, "Well, you hood."
(Via Talking Points Memo)