Inexplicably means for reasons nobody can account for. It happened, and no explanation is available.
It usually sits before a verb or opens a clause, so the door was inexplicably unlocked, or inexplicably, nobody had been told. The word is a strong claim, since it rules out every explanation rather than the obvious ones, which is why it often really means nobody has looked into it. It is a favourite of narrative writing, where it flags a mystery without having to describe one. Unaccountably is the near synonym.
1adverbinexplicablyIn a way that cannot be explained.
“The train was inexplicably held outside the station for an hour.”
Latin
inexplicābilis
Middle French
inexplicable
Middle English
inexplicable
English
inexplicable
English, today
inexplicably
From inexplicable + -ly. inexplicable comes from Middle English inexplicable, from Middle French inexplicable, from Latin inexplicābilis, from in- + explicābilis. by surface analysis, in- + explicable.
Etymology from Wiktionary, CC BY-SA