Inexplicable means impossible to account for. Not merely surprising — there is no explanation available at all.
It describes events, decisions and absences, so an inexplicable delay, an inexplicable decision and an inexplicable failure to act are all standard. It is a strong word, since claiming something is inexplicable rules out every explanation rather than just the obvious ones, which is why it often really means nobody has bothered to look. The adverb inexplicably usually sits before a verb. Unaccountable is its close relative.
1adjectiveinexplicableImpossible to explain or account for.
“For some inexplicable reason the door was unlocked.”
Latin
inexplicābilis
Middle French
inexplicable
Middle English
inexplicable
English, today
inexplicable
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