The cardboard box is an item that is spawned by all of Snake's taunts in Super Smash Bros. Brawl. By using any of his taunts, Snake crouches under a cardboard box, which he removes after a short time (or when a button is pressed). If the box hits an opponent after being removed, it does minimal damage (1%-2%) and low knockback. Any character can pick up the box before Snake removes it; they will lift the box like a heavy item and automatically throw it forwards (unless another direction is specified). Donkey Kong's unique heavy-item mechanics allow him to carry the box around and throw it at will, however. Removing the box does not interrupt the taunt's duration.
Role in Subspace Emissary
The cardboard box appears inside the Halberd's cargo hold before Snake is revealed to be in it. In a later cut scene, Snake sheds the box, saying "Kept you waiting, huh?" Later, while Snake is sneaking around the ship, he hides under the box again to avoid detection by Lucario and Meta Knight. However, Lucario uses its aura powers to detect Snake and removes the cardboard box, resulting in the trademark "!" appearing.
Snake must often sneak around enemy territory undetected in the Metal Gear series. One reoccurring way to avoid detection by guards in industrial areas is to hide under a cardboard box in order to not alert their attention. However, Snake must be careful not to be caught moving while under the box. Its first appearance was in Metal Gear and calling Big Boss while within the room where the player obtains the box leads to him mockingly asking if the player is moving house. Ironically, Big Boss himself had made use of it during Snake Eater, back when he was known as Naked Snake.
The design of boxes in the Metal Gear series varies depending on their location. As a result, the box in Brawl has no real origin and is instead composed entirely of meta-elements: it has the Super Smash Bros. logo with "Smash Bros." printed on the long sides and a "!" on the short sides. Interestingly, these elements are printed upside-down (so they appear rightside-up as Snake wears the box upside-down).
Snake can be seen hidden inside of the trophy.