The main, traditional meaning of fourreau was for a child's dress. In Garsault's L'Art de la Lingere, infants are described as wearing various pieces of the layette until they reach three, at which point the girls are put into shifts and jacquettes, while boys wear the fourreau until they're breeched at four or five. However, in Galerie des Modes, the caption-writer is consistent in describing the young fourreau-wearers as girls. (It's possible that this is not contradictory - Garsault was published in 1761, and a shift in word- or dress-usage may have occurred over the following two decades.)
|32e Cahier, 5e Figure, 1780|
This sort of echoing existed in the adult woman's fourreau as well. The Cabinet des Modes reported that "gowns and fourreaux à l'Anglaise, à la Turque, à la Janseniste, à la Circassienne, are still in fashion. When a Lady is in a green fourreau, à la levite ..."
The adult version of the fourreau appears in the fashion plates in 1784, around the same time as the chemise gown, and the plates seem to show that - like the child's fourreau and the chemise - it has a closed skirt.
|39e Cahier (bis 2), 3e Figure, 1784|
So far, I haven't been able to determine how late the fourreaux were worn - 1790 is the latest I've come across the term at the moment - but they seem to be generally part of the pre-Neoclassical Anglomania period. To my knowledge, there are no extant fourreaux in collections online, which would suggest that they were less common outside of the pages of fashion magazines. However, many portraits of the 1790s show very plain gowns that could possibly be fourreaux.
(As of this point, I haven't translated much of Cabinet des Modes or Journal de la Mode et du Gout, and given their time of publication it seems very likely that the fourreau will come up in more fashion plates and descriptions. This page may be updated later!)