Source : Webster's Revised Unabridged Dictionary (1913)

Excruciating \Ex*cru"ci*a`ting\
   Torturing; racking. ``Excruciating pain.'' --V. Knox.
   ``Excruciating fears.'' --Bentley -- {Ex*cru"ci*a`ting*ly},
   adv.

Source : WordNet®

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