The Collaborative International Dictionary of English v.0.48:

Excruciating \Ex*cru"ci*a`ting\
   Torturing; racking. "Excruciating pain." --V. Knox.
   "Excruciating fears." --Bentley -- Ex*cru"ci*a`ting*ly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

excruciatingly
    adv 1: in a very painful manner; "the progress was agonizingly
           slow" [syn: agonizingly, excruciatingly,
           torturously]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "excruciatingly":
   abominably, agonizingly, awfully, baldly, balefully, bitterly,
   blatantly, brashly, confoundedly, cruelly, damnably, deadly,
   deathly, deplorably, deucedly, distressfully, distressingly,
   dolorously, dreadfully, egregiously, excessively, exorbitantly,
   extravagantly, flagrantly, frightfully, grievously, harrowingly,
   heartbreakingly, hellishly, horribly, improperly, inexcusably,
   infernally, inordinately, intolerably, lamentably, miserably,
   nakedly, openly, painfully, pathetically, piteously, pitiably,
   ruefully, sadly, shatteringly, shockingly, something awful,
   something fierce, sorely, staggeringly, terribly, torturously,
   unashamedly, unbearably, unconscionably, unduly, unpardonably,
   woefully