Source : Webster's Revised Unabridged Dictionary (1913)

Grimly \Grim"ly\, a.
   Grim; hideous; stern. [R.]

         In glided Margaret's grimly ghost, And stood at
         William's feet.                          --D. Mallet.

Grimly \Grim"ly\, adv.
   In a grim manner; fiercely. --Shak.

Source : WordNet®

grimly
     adv : in a grim implacable manner; "he was grimly satisfied"