very silly or stupid in English:

His behaviour has been preposterous recently

It was fun escapism and we emerged from the cinema in a big West London shopping mall laughing about some of more preposterous stunts we had just witnessed.

don’t be preposterous!

The whole thing still strikes me as totally preposterous

To me, at the time, it sounded preposterous because I didn’t think anybody would buy a Pet Rock.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well- warmed, and well-fed.