If something is inconceivable, it doesn’t seem possible, it’s hard to imagine, or it can’t be true. It might seem inconceivable that a society would choose to pay bankers more than teachers, but it’s true.
When an idea or event is inconceivable, it's so outlandish, amazing, or horrible that it’s hard to believe. The horrible acts committed by the Nazis during the Holocaust are inconceivable on many levels, yet we know they occurred. Some things are inconceivable because they really wouldn't happen — for example, it's inconceivable that you would actually eat your hat.