dungeon (n.) Look up dungeon at Dictionary.com
c.1300, "great tower of a castle," from Old French donjon "great tower of a castle" (12c.), from Gallo-Roman *dominionem, from Late Latin dominium, from Latin dominus "master" (of the castle; see domain). Sense of "castle keep" led to "strong (underground) cell" in English early 14c. The original sense went with the variant donjon.