inconceivable or unimaginable; extremely improbable in a way that goes against common sense
Nothing is unthinkable, nothing impossible to the balanced person, provided it comes out of the needs of life and is dedicated to life's further development - Lewis Mumford.
emphasis If you say that something is unthinkable, you are emphasizing that it cannot possibly be accepted or imagined as a possibility. Her strong Catholic beliefs made abortion unthinkable. The unthinkable is something that is unthinkable. Edward VIII had done the unthinkable and abdicated the throne
You can use unthinkable to describe a situation, event, or action which is extremely unpleasant to imagine or remember. This place is going to be unthinkable without you