While discussing preparations for the ball, it transpires that Rory will be wearing a crinoline under her dress, and Dean will need to wear a cummerbund.
A crinoline is another word for a hoop skirt, previously discussed.
A cummerbund is a broad sash worn around the waist, these days usually with a dinner jacket or tuxedo. Originating in Persia, they were adopted by British officers stationed in India, and adopted as an alternative to the waistcoat. A cummerbund is worn for black tie events in North America.