Fr: Polish gown. A dress with the skirt hitched up at the sides and in back.
Just to make things difficult for the likes of us, there are two differnet gowns of that name. Actually, there are three, but the third kind belogns to the 19th century.
The first and best known robe à la polonaise came into fashion during the early 1770s. Its most prominent feature is that the robe skirt is hitched up in two places between sides and back. It is often seen with a short (sometimes less than ankle-length) jupe. Both jupe and robe usually had flounces round the hem. The sleeves were elbow-length. The name is said to be derived from the fact that in 1772, Poland was divided into three parts, just as the skirt was.
There are two prominent sub-styles, the robe à la circassienne and the robe à la turque. The skirt of the circassienne was hitched up well above the jupe hem so that it formed three semicircular drapes. As for the turque, there are two styles of that name.
The second, lesser known polonaise appeared late in 1794 or early in 1795. It is a variation of the later robe à la turque. I you feel confused now, what with the turque being a variation of the polonaise and the other way round, you'll understand why it is so difficult to define the major late 18th century robe styles: There was a babylonian confusion of names, changing every half year or so, one being a sub-style of the other, the only difference often being in one little detail.
So, there is a 1770s polonaise that has a variation called robe à la turque, and a 1790s turque that has a variation called robe à la polonaise. The 1770s turque differed from the polonaise by having a train. As for the 1790s turque and polnaise, see robe à la turque.
polonaise (from the Costumer's Manifesto)