Source : Webster's Revised Unabridged Dictionary (1913)

Solemnly \Sol"emn*ly\, adv.
   In a solemn manner; with gravity; seriously; formally.

         There in deaf murmurs solemnly are wise. --Dryden.

         I do solemnly assure the reader.         --Swift.

Source : WordNet®

solemnly
     adv : in a grave and sedate manner; "the judge sat there solemnly"