The phrase "a preposterous thing" is correct and usable in written English.
It can be used to describe something that is absurd, ridiculous, or contrary to reason.
Example: "To suggest that we could solve the problem overnight was a preposterous thing to say."
Alternatives: "an absurd notion" or "a ridiculous idea".
Who would do such a preposterous thing?
A preposterous thing happened two days ago.
It was such a preposterous thing to say.
"The fact that the law says it is illegal to discuss homosexuality as a normal practice to anybody under the age of 18 is a preposterous thing".
He added that her plan for her party members to participate in the march across the Punjab to Islamabad was "a preposterous thing to do".
Albeit... in 1707, when John Leverett became the leading candidate for ascension to Harvard's Presidency, Cotton Mather railed, "To make a lawyer, and one who never affected the study of divinity, a president for a College of Divines will be a very preposterous thing, a thing without precedent".
The preposterous thing was a gift from the Philippine government.
I could avert my eyes from pastel bunnies hatching out of eggs or doing some other preposterous thing, if I didn't know that around Easter too many rabbits are bred by opportunists and purchased on impulse by well-meaning parents who approach them like stuffed toys.
Ms. Hunter and the Iranian president have a disconcerting way of saying preposterous things in a calm, complacent tone; both are serenely impervious to contradiction or sarcasm.
Of all the preposterous things in HONEYMOON IN VEGAS (1992), a loopy film whose climax involves skydiving Elvis impersonators, the most preposterous is that James Caan, as a gambler who wants to steal Sarah Jessica Parker from Nicolas Cage, also steals the movie.
Chinese Communist officials constantly revile His Holiness in the most scathing and inappropriate language, and while conducting talks with his representatives, simultaneously denying that they are doing so, speak of him insultingly, and accuse him of all sorts of preposterous things.
