Unbelievable—If you don't have the EMF song stuck in your head right now, you're lying.
Far-Fetched—Like you need to be Reed Richards (Mr. Fantastic) to accept this.
Improbable—Made popular by that Captain Jack Sparrow meme.
Implausible—Like improbable, but significantly more educated.
Unlikely—Used to be a great word, until Facebook redefined 'unlike.'
Incredible—Like the Hulk.
Inconceivable—Like the Sicilian
Unthinkable—Like a war crime.
Unimaginable—Okay, now you're not giving me enough credit.
Unconvincing—Are you calling me a liar?
Mind-Blowing—I think non-lethal explosions contained within the cranium are unlikely.
Preposterous—You ARE calling me a liar.
Outlandish—It's like a lie dressed up in a Scottish Kilt.
Cockamamie—Unbelievable with a hint of crazy.
Questionable—I don't think you're an idiot, but your actions aren't helping you.
Doubtful—I am full of doubt.