An Etymological Dictionary of the German Language/Frauenzimmer
Frauenzimmer, n., ‘woman,’ from late MidHG. vrouwenzimmer, n., ‘women's apartment’; the connecting link in meaning is collective, ‘the body of women residing in its own apartments, the female inhabitants of the gynæceum,’ also ‘retinue of a lady of high rank,’ just as Hof (court) is used collectively of ‘the people at court.’ “The application of a collective term to an individual” is analogous to the use of Bursche and Kamerad; the modern sense dates from the beginning of the 17th cent.