inconceivable. adjective. not conceivable; unthinkable; unbelievable.
"You keep using that word; I do not think it means what you think it means." (Inigo Montoya)
Princess Bride fans, rejoice! This is obviously far from a new or obscure word, but it's a damn good one that we should all be using more often! This one's usage is pretty straight forward. You wanna describe something as beyond belief or something that should be well outside the boundaries of reality, you may refer to it as inconceivable:
I still find it inconceivable how she chose to start dating him...
She drew up this absolutely inconceivable plan for moving the television downstairs.
Stan woke up with inconceivably bad breath.
Allegedly the word also works as a one-word interjection; usage in this manner is for comedic value at best. Otherwise it probably doesn't fit how you think it fits.
(noun form: inconceivability, inonceivableness [apparently]; adverb form: inconceivably.)
