the unthinkable has happened.
It was unthinkable to leave him behind. = Era impensable dejarlo atrás.
Such a mistake would have been unthinkable before. = Un error así habría sido inconcebible antes.
The idea seemed unthinkable at the time. = La idea parecía impensable en ese momento.
unthinkable: significa impensable, inconcebible, algo que no se puede imaginar porque parece demasiado extremo, improbable o inaceptable.