Rabbi Yehuda Herzl Henkin, shlita, mentions that quotation on page 37 of his essay "Contemporary Tseni'ut" (Tradition Fall 2003), disagreeing with it quite sharply. He says it's found in Rabbi Falk's Oz VeHadar Levusha page 40, but I don't know if Rabbi Falk cites anyone on it. So that's a first step.
(I can't help but mention that the family's biography of Rav Moshe Feinstein prefacing Igros Moshe vol. 8 describes his wife Sima. Today if you described how a rebbetzin dressed, the only word you could use would be "tznius." Instead, it says she "was always dressed meticulously and with excellent taste.")