The bonesack is a large brown sack filled with bones worn in the cape slot. It is a reward from completing the Rag and Bone Man wish list miniquest. Contrary to popular belief it does not store bones. It can be switched for a Ram skull helm by talking to the Rag and Bone Man again. This item can be enchanted to a Bonesack(e) after completing the Fur 'n' Seek wishlist.
The bonesack's examine info is a reference to the song "Love Shack" by the B-52s.
The "talking" bonesack is a reference to the H. C. Andersen fairy tale, "Little Klaus and Big Klaus," in which only little Klaus is able to "communicate" with a sack containing his grandmother's bones.