Compound of 乳(にゅう) (nyū, “breast”) + 輪(りん) (rin, “ring”).
乳(にゅう)輪(りん) • (nyūrin)
乳輪 or 乳輪 or 乳輪 • (yuryun or McCune-Reischauer: yuryun or Yale: yulyun) (hangeul 유륜)