Pronunciation: (bī'swing"), [key]
(of a garment) made with a deep pleat starting at the back waistline or belt and extending up to the shoulder on each side, to avoid constriction when the wearer's arms are extended.
a garment made with such pleats at the back, esp. a jacket.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.