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The phenomenon where the bottom bun of a burger mysteriously disappears faster than the top bun. The eater's thumbs are precariously placed on a postage-stamp sized "under-bun", while the top bun is still the same size as the burger.
Your thumbs are covered with ketchup because you've got a bad case of under-bun syndrome.
by ivegothookworms June 29, 2010
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