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general::
1. A wretched prison cell or other place of confinement. For example, The punishment is solitary confinement, known as the black hole. This term acquired its meaning in 1756 with the event known as the Black Hole of Calcutta. On the night of June 20, the ruler of Bengal confined 146 Europeans in a prison space of only 14 by 18 feet. By morning all but 23 of them had suffocated to death. Although historians since have questioned the truth of the story, it survives in this usage. 2. A great void or abyss. For example, Running a single small newspaper ad to launch a major campaign is useless; it amounts to throwing our money into a black hole. This usage alludes to a region, so named by astronomers, whose gravitational field is so intense that no electromagnetic radiation can escape from it. [Late 1970s]
American Heritage Idioms