If you want to call someone foolish while sounding smart yourself, the adjective asinine is your go-to word.
Asinine derives from the Latin asinus. Guess what that means? Bonus: Asinine takes stupid up a notch. There's someone so delightful in the double-edged quality of asinine behavior. Talking trash about your boss is stupid. Talking trash about your boss in an email they're cc'd on...that's asinine.