Meaning of “unaccountable” - Learner’s Dictionary

unaccountable

adjective uk us /ˌʌnəˈkaʊntəbl/
SURPRISING

impossible to explain:

For some unaccountable reason, I've got three copies of the same book.
PERSON

not having to give reasons for your actions or decisions

unaccountably adverb

(Definition of “unaccountable” from the Cambridge Learner’s Dictionary © Cambridge University Press)