Preposterous means so ridiculous or absurd that it's almost laughable. When something is preposterous, it goes completely against common sense or reason in such an extreme way that it seems impossible to believe.

If your friend claimed she could jump over the school building, that would be a preposterous claim. If someone suggested doing homework by training squirrels to write with tiny pencils, that's a preposterous idea. The word captures that special quality of being so far from reality that you can barely take it seriously.

Notice that preposterous is stronger than just “silly” or “wrong.” A math answer can be wrong without being preposterous, but claiming that 2 + 2 = 847 would be preposterous. The word often appears when someone makes an outrageous suggestion or accusation: “That's preposterous!” someone might exclaim when hearing an impossible story.

You might encounter preposterous theories in old science books, like the once-accepted but now obviously preposterous idea that Earth is the center of the universe. What seems reasonable in one era can later appear preposterous once we know better.