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]
Proto-Indo-European root, the stem of demonstrative pronoun meaning "this."
It is the hypothetical source of/evidence for its existence is provided by: Greek ekeinos "that person;" Latin cis "on this side," citra (adv.) "on this side;" Old Church Slavonic si, Lithuanian is, Hittite ki "this;" Old English hider, Gothic hidre "hither."
early 15c., from Old French vexer "vex, harass" (14c.), from Latin vexare "to shake, jolt, toss violently;" figuratively "attack, harass, trouble, annoy," from vexus, collateral form of vectus, past participle of vehere "to draw, carry" (from PIE root *wegh- "to go, move, transport in a vehicle"). Related: Vexed; vexing.
The weakened or transferred meaning "to trouble in body or mind, harass, distress," is attested from 1530s. Related: Afflicted; afflicting.
1550s, "to trouble, afflict, harass," later "to beat, thrash" (1560s), of uncertain origin. Perhaps related to French troncer "to cut, cut off a piece from," from tronce "piece of timber," from Old French tronc (see trunk (n.1)). Related: Trounced; trouncing.
1739, "to scold," a word of unknown origin; perhaps related to Danish dialectal rag "grudge." Compare bullyrag, ballarag "intimidate" (1807). Weakened sense of "annoy, tease, harass roughly" is student slang, by 1808. Related: Ragged; ragging.