General term for a shape such as a dango, empty triangle etc. that fails to work well because it is inefficient, or suffers from shortage of liberties, has too many cutting points, too little eye shape, and so on. It is particularly problematic earlier on in the game, as it gives the opponent weaknesses to aim at.
Often a tesuji exists to exploit the weakness(es) of bad shape. A weird shape that appears in actual play is the B2 Bomber.