dead weight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The unrelieved weight of a heavy, motionless mass.
• noun An oppressive burden or difficulty.
• noun The fixed weight of a structure or piece of equipment, such as a bridge on its supports.

from The Century Dictionary.

• noun In railroading, the weight of a car as distinct from its load (called the live or paying weight) whether it is a freight-car, a passenger-car, or a stock-car.
• noun A heavy or oppressive burden; a weight or burden that has to be borne without aid or without compensatory advantage.
• noun A name given to an advance by the Bank of England to the government on account of half-pay and pensions to retired officers of the army and navy.
• noun Nautical, the lading of a vessel when it consists of heavy goods; that part of the cargo, as coal, iron, etc., which pays freight according to its weight, and not to its bulk.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable unremitting heavy weight that does not move
• noun idiomatic that which is useless or excess; that which slows something down

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an oppressive encumbrance
• noun a heavy motionless weight