expression, pre-noun adjectival
inexcusable; unthinkable; unacceptable
Used to describe something that should not happen or cannot be tolerated, often with a strong moral or logical judgment. Functions as a pre-noun adjectival modifying a noun.
彼の行動はあり得べからざる過ちだ。
His actions are an inexcusable mistake.
expression, pre-noun adjectival
utterly unexpected; totally unforeseen
Describes an event or situation that is completely surprising and beyond what one could have imagined. Also pre-noun adjectival.
あり得べからざる事態が発生した。
An utterly unexpected situation has occurred.
あり得ない is the modern, colloquial equivalent meaning 'impossible' or 'unthinkable', while あり得べからざる is a formal, literary expression used mainly in writing.
A classical Japanese expression combining あり (to be) + 得 (can) + べからざる (archaic negative potential form of べし, meaning 'should not be able to'). Literally 'should not be able to exist', it fossilized as a formal pre-noun adjectival.