1610s, "to lay waste, devastate" (obsolete); 1620s, "to vex by repeated attacks," from French harasser "tire out, vex" (16c.), which is of uncertain origin; possibly from Old French harer "stir up, provoke; set a dog on" (according to Watkins, from Frankish *hara "over here, hither," from Proto-Germanic *hi‑, from PIE *ki-, variant form of root *ko-, the stem of demonstrative pronoun meaning "this"), and perhaps blended with Old French harier "to harry, draw, drag" [Barnhart]. Related: Harassed; harassing.
Harass, as applied to mind or body, suggests the infliction of the weariness that comes from the continuance or repetition of trying experiences, so that there is not time for rest. Torment implies the infliction of acute pain, physical or mental, and is frequently used in the sense of harassing by frequent return. [Century Dictionary, 1897]