The word "preposterous" means something that is extremely ridiculous, absurd, or completely illogical. It describes an idea, statement, or action that is so outrageous that it challenges common sense or reason. Full definition
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)