The Word of the Day comes from "Bone China" by Laura Purcell.
In the novel the word is used to describe the dress that a character is wearing.
The robe à la polonaise or polonaise is a woman's garment of the later 1770s and 1780s or a similar revival style of the 1870s inspired by Polish national costume, consisting of a gown with a cutaway, draped and swagged overskirt, worn over an underskirt or petticoat. From the late 19th century, the term polonaise also described a fitted overdress which extended into long panels over the underskirt, but was not necessarily draped or swagged.
However, it is also a type of dance.
Polonaise, Polish polonez, in dance, dignified ceremonial dance that from the 17th to 19th century often opened court balls and other royal functions. Likely once a warrior’s triumphal dance, it was adopted by the Polish nobility as a formal march as early as 1573 for the coronation of Henry of Anjou as king of Poland. In its aristocratic form the dancers, in couples according to their social positions, promenaded around the ballroom with gliding steps accented by bending the knees slightly on every third step. Polonaise music is in 3/4 time. The dance was used as a musical form by such prominent composers as Beethoven, Handel, Mussorgsky, and Chopin.