Lagging Tail is a held item that appears in the Generation IV Pokémon games. When held by a Pokémon it causes the Pokémon to strike last in the priority bracket (basically it does the opposite of the Quick Claw item, except that it always works).
It does not increase the power of Gyro Ball like the Iron Ball does, since it alters the priority of the user's moves rather than lowering the users speed.