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), from domus "house" (from PIE root *dem- "house, household"). Sense of "castle keep" led to "strong (underground) cell" in English early 14c. The original sense went with the variant donjon.
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