unmentionable thing

Hers was one of those still existing minds which read immorality into all attempts to break away from her own strict set of conventions, especially when it was in the direction of beautifying a bed, to her, of course, an unmentionable thing.

He drew Jurgis out, and heard all about his life all but the one unmentionable thing; and then he told stories about his own life.

As is common in most modern discussions the unmentionable thing is the pivot of the whole discussion.