Rashi explains the second phrase in his commentary to the parallel section in Berakhot 24a, s.v. takhshitin shebifnim: "Kumaz — a defuss of the uterus that they would make for their daughters, and they would pierce the walls of the uterus as one pierces the ears and insert it so that men would not have relations with them". In other words, according to Rashi, kumaz means a "chastity belt", and Rabbah hints at this later on in the sugya in Shabbat, using a notarikon: "KuMaZ — Kan Mekom Zima", here is a place of lewdness".
Source: wiktionary