Is it ensure, insure, or assure?
c.1300, "great tower of a castle," from Old French donjon "great tower of a castle" (12c.), from Gallo-Romance *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.
different from the ordinary prison in being more severe as a place of punishment. Like the Roman inner prison (Acts 16:24), it consisted of a deep cell or cistern (Jer. 38:6). To be shut up in, a punishment common in Egypt (Gen. 39:20; 40:3; 41:10; 42:19). It is not mentioned, however, in the law of Moses as a mode of punishment. Under the later kings imprisonment was frequently used as a punishment (2 Chron. 16:10; Jer. 20:2; 32:2; 33:1; 37:15), and it was customary after the Exile (Matt. 11:2; Luke 3:20; Acts 5:18, 21; Matt. 18:30).