The phrase "a preposterous idea that" is correct and usable in written English.
You can use it to introduce a statement or opinion that is considered absurd or ridiculous.
Example: "She proposed a preposterous idea that we should all quit our jobs and travel the world without any plans."
Alternatives: "an absurd notion that" or "a ridiculous suggestion that".
The Harvard institute, which has "recruited leading researchers from around the world," explores the "preposterous idea" that giving placebos deliberately as medicine might have a place in cures.
As a piece of theatre, it's perfect, brilliantly edited and satisfyingly complete, but it's predicated on a single ugly truth: the preposterous idea that a homely, middle-aged woman could possibly have talent.
Speaking of this preposterous idea that a woman needs a man: Who thought this horribly sexist Batman T-shirt was a good idea?
And then there's the preposterous idea that "viral videos" should be a concern at all.
It floated the preposterous idea that Mr. Lin might switch nationalities to play for Communist China — preposterous because he is an American, and doubly absurd because his family is from Taiwan.
"This preposterous idea that corporations will solve the world's development challenges is so out of touch [with] reality that it would be comical if it were not for its disastrous consequences," she said.
He evidently got Trump to nod along to the preposterous idea that Russian investigators should be allowed to observe interrogations of present and former senior U.S. officials on bogus political charges.
The good doctor continues to provide good copy, although the preposterous idea that his wife Mandy could this week be selected by the administrators as the best bet to rescue the Bradford Bulls is best ignored.
What makes "Prime Time Love" such a hoot isn't merely the image of a grown man too lazy to walk to the kitchen, but also the even more preposterous idea that someone married to the one person ever to play in both the Super Bowl and the World Series would expect an equitable division of household labor.
