Peach's Extra Dress
From the Super Mario Wiki
Peach's Extra Dress is a key item in Mario & Luigi: Superstar Saga. At Beanbean Castle, before Mario and Luigi head for Joke's End to exchange the Beanstar for Princess Peach, who is kidnapped by Bowletta, Toadsworth worries that Princess Peach might have damaged her dress while escaping, and comes to give this item to the Mario bros. Later in the game, Bowletta takes away the Beanstar while refusing to give out Princess Peach, and Mario comes up with a plan. Mario has Luigi use this item to dress up like Princess Peach, and fools Bowletta into thinking that the Princess Peach they have caught is fake. Bowletta then takes Luigi away, dropping the real Princess Peach. On the Repaired Koopa Cruiser, Fawful finds out that the new Princess Peach has a mustache, and fired at her head to find out that the princess is really Luigi. After Luigi flees to the next room, he takes this dress off.
When Luigi escapes from the Koopa Cruiser, he uses this dress as a parachute.