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"), so called probably for its commanding position or strength. Sense of "castle keep" led to that of "strong (underground) cell" in English early 14c. The original sense went with the variant donjon.
