Preposterous means something that is so ridiculously untrue or impossible, it is laughable. It suggests a claim or idea that is completely absurd and beyond belief.
Usage examples
Their claim that aliens built the pyramids was preposterous.
The idea of a flying pig is preposterous.
It is preposterous to think that the Earth is flat.
The proposal to ban all forms of transportation except bicycles is preposterous.
His preposterous attempt to juggle flaming torches ended predictably with a visit to the emergency room.
The notion that birds can swim underwater like fish is utterly preposterous.
The film's plot was so preposterous that it was hard to suspend disbelief and enjoy the movie.
The politician's preposterous promise to solve all the country's problems overnight was met with skepticism.
It is preposterous to assume that all students have the same learning style.
The argument that dogs are capable of speaking multiple languages is preposterous.
The very notion of it seemed as preposterous as, say, hauling a load of Pennsylvania coal into Newcastle harbor. (si.com)
That just seems preposterous for a momma on a budget. (babygaga.com)
But the report, loaded with inaccuracies and distortions, ultimately proved that a study commissioned by a teachers union is about as valid as preposterous claims made by a 3 am TV pitchman hawking wrinkle cream. (californiapolicycenter.org)