also hour-glass, instrument for measuring time, 1510s, from hour + glass (n.). Used 19c. in a variety of technical and scientific senses to describe the shape; in reference to women's torsos by 1897.
Men condemn corsets in the abstract, and are sometimes brave enough to insist that the women of their households shall be emancipated from them; and yet their eyes have been so generally educated to the approval of the small waist, and the hourglass figure, that they often hinder women who seek a hygienic style of dress. [Mary Ashton Rice Livermore, "The Story of My Life," 1898]