The Collaborative International Dictionary of English v.0.48:

Harrow \Har"row\, v. t. [imp. & p. p. Harrowed
   (h[a^]r"r[-o]d); p. pr. & vb. n. Harrowing.] [OE. harowen,
   harwen; cf. Dan. harve. See Harrow, n.]
   1. To draw a harrow over, as for the purpose of breaking
      clods and leveling the surface, or for covering seed; as,
      to harrow land.
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            Will he harrow the valleys after thee? --Job xxxix.
                                                  10.
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   2. To break or tear, as with a harrow; to wound; to lacerate;
      to torment or distress; to vex.
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            My aged muscles harrowed up with whips. --Rowe.
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            I could a tale unfold, whose lightest word
            Would harrow up thy soul.             --Shak.
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WordNet (r) 3.0 (2006):

harrowing
    adj 1: extremely painful [syn: agonizing, agonising,
           excruciating, harrowing, torturing, torturous,
           torturesome]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "harrowing":
   acute, afflictive, agonizing, alarming, atrocious, biting,
   chilling, consuming, cramping, cruel, daunting, desolating,
   disconcerting, dismaying, disquieting, distressing, disturbing,
   excruciating, frightening, gnawing, grave, griping, hard, harsh,
   heartbreaking, heartrending, heartsickening, heartwounding,
   horrible, horrifying, hurtful, hurting, nerve-racking, painful,
   paroxysmal, piercing, poignant, pungent, racking, rending, severe,
   sharp, shooting, spasmatic, spasmic, spasmodic, stabbing, stinging,
   tearing, terrifying, tormenting, torturing, torturous, traumatic,
   upsetting, vexing, worrisome, worrying