Definition of unaccountably adverb from the Oxford Advanced Learner's Dictionary

 

unaccountably

 adverb
adverb
BrE BrE//ˌʌnəˈkaʊntəbli//
 
; NAmE NAmE//ˌʌnəˈkaʊntəbli//
 
(formal)
 
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in a way that is very difficult to explain; without any obvious reason synonym inexplicably He has been unaccountably delayed. She felt suddenly and unaccountably cheerful.
See the Oxford Advanced American Dictionary entry: unaccountably