In Magic, a cube is a collection of cards that is handpicked to make a good draft. A cube draft is a type of limited format that uses a cube to draft from. But what does "cube" mean? Does it refer to a collection of cards in the general sense, or is it Magic-specific terminology? Where did this term originate?
I believe the first cube was kept in a box that was a cube.
I wouldn't call that explanation definitive, but a quick Google search demonstrates that there are Magic players who would back up Rosewater's memory.