The Collaborative International Dictionary of English v.0.48:

Watteau back \Watteau back\
   The back of a woman's gown in which one or more very broad
   folds are carried from the neck to the floor without being
   held in at the waist, while the front and sides of the gown
   are shaped to the person and have a belt or its equivalent.
   [Webster 1913 Suppl.]