Bag of Holding

Bag of Holding, p.n., D&D-ism: from the Advanced Dungeons & Dragons Dungeon Masters Guide, a magical leather pouch or bag that is much larger on the inside than on the outside, and therefore useful for carrying things. [GJ1]
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Addendum: Very popular among fantasy gamers as a way to skirt encumbrance rules. See Gygax Slices. [GJ1]