The Knothole Skirt is obtained from the Box of Secrets on Knothole Island. To gain access to Knothole Island, you must have the Knothole Island DLC. The hint on the package says: "A Sexy Garment". This is possibly said because of how short the skirt is. In order to get the skirt you must trade in the book Marriage and How to Survive It. This book is obtainable from Fiction Burns (the bookstore) and the Jeweller in Bowerstone Market.