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An extraordinarily large pair of breasts. So much so, that they have ascended beyond the point of simply being a "rack", and have in fact raised to the level of a "shelving unit."

Often used as a way of expressing the sheer monstrous volume of fat contained within the Mammory Glands of a Homosapien (Human) Female.
Guy #1: Man, check out the shelving unit on that!
Guy #2: I bet you could even rest your elbows on them to light your smoke.
Guy #1: My dream girl.
by Edward Fitzgerald December 21, 2010