the mind > mental capacity > intelligibility > unintelligibility > inexplicableness > [adverb]
unaccomptibly1678
unaccountably1694
unexplainedly1811
undecipherably1847
unexplainably1899
inexplicably1903
mystifyingly1937
1811 L.-M. HawkinsCountess & Gertrude II. xlii. 366 These insular situations,..where nothing can occur unexpectedly and unexplainedly, without..carrying an inflammable train.