inconceivable◼◼◼(unable to be conceived, unbelievable) adjective [UK: ˌɪnk.ən.ˈsiː.vəb.l̩] [US: ˌɪnk.ən.ˈsiː.vəb.l̩]It is inconceivable to me that he would do such a thing. = Il m'est inconcevable qu'il fasse une chose pareille.
unthinkable◼◼◻(inconceivable or unimaginable; extremely improbable in a way that goes against common sense) adjective [UK: ʌn.ˈθɪŋk.əb.l̩] [US: ʌn.ˈθɪŋk.əb.l̩]