That moment was recounted in Geithner's new memoir, Stress Test. It happened in the 1980s when Geithner and D'Souza were undergraduate students at Dartmouth College. D'Souza, then an editor of the conservative Dartmouth Review, had overseen the newspaper publishing a list of members of Dartmouth College's Gay Straight Alliance which effectively outted a number of students who hadn't publicly said they were gay.
According to Geithner in his memoir, as reported by Buzzfeed, he viewed the list as "McCarthy-style." When Geithner ran into D'Souza at a coffee shop he asked the conservative writer "how it felt to be such a dick."
Watch Geithner recount the encounter in an interview with Vox.com's Ezra Klein: This post was updated.