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.

c. 1400, "to vex, irritate," probably a back-formation from crabbed. The notions of "bad-tempered, combative" and "sour" in the two nouns crab naturally yielded a verb meaning of "to vex, irritate," later "to complain irritably, find fault" (c. 1500). As "to fish for crabs" from 1650s (implied in crabbing). The noun meaning "sour person" is from 1570s.

"vex, mortify," 1660s (implied in chagrined), from chagrin (n.). Related: Chagrining.

late 15c. (Caxton), "infest with disease or other natural calamity," from Middle Dutch plaghen, from plaghe (n.) "plague" (see plague (n.)). The sense of "vex, harass, bother, annoy" is recorded from 1590s. Related: Plagued; plaguing.

late 14c., distemperen, "to disturb, upset the proper balance of," from Old French destemprer and directly from Medieval Latin distemperare "vex, make ill," literally "upset the proper balance (of bodily humors)," from dis- "un-, not" (see dis-) + Latin temperare "mingle in the proper proportion" (see temper (v.)). Related: Distempered.

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]