A dark underground prison, especially in a castle or fortress.
From Old French 'donjon' meaning tower or keep, from Latin 'dominio' meaning domain. Originally referred to the keep of a castle, then came to mean underground prisons.
Medieval dungeons were often so terrible—dark, wet, disease-ridden—that they were literally considered a death sentence; sometimes being 'thrown in the dungeon' meant you wouldn't survive, making it as much an execution as a prison.
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