Formal:
* Bête: This is the most literal translation and means "stupid" or "foolish." It's generally used in a more formal context.
Informal:
* Niais: This means "silly" or "foolish" and is more commonly used in informal settings.
* Absurde: This means "absurd" and can be used to describe something silly or nonsensical.
* Con: This is a slang term meaning "stupid" or "silly."
The best word to use will depend on the specific situation and the level of formality.