The Collaborative International Dictionary of English v.0.48:

Solemnly \Sol"emn*ly\, adv.
   In a solemn manner; with gravity; seriously; formally.
   [1913 Webster]

         There in deaf murmurs solemnly are wise. --Dryden.
   [1913 Webster]

         I do solemnly assure the reader.         --Swift.
   [1913 Webster]

WordNet (r) 3.0 (2006):

solemnly
    adv 1: in a grave and sedate manner; "the judge sat there
           solemnly"

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "solemnly":
   bleakly, ceremoniously, decorously, demurely, dismally, drearily,
   earnestly, formally, funereally, gloomily, grandly, gravely,
   grimly, heavily, imposingly, in due form, in set form, loftily,
   magisterially, majestically, nobly, pro forma, regally,
   ritualistically, ritually, royally, sadly, sedately, seriously,
   sober-mindedly, soberly, sobersidedly, somberly, sombrously,
   staidly, stately, thoughtfully, venerably, with dignity,
   worthily