The phrase "silly idea" means a thought or suggestion that is not very smart, logical, or serious. It often refers to something that is foolish or not worth considering. Full definition
Maybe they they think this will show that this whole silly idea about humans causing the earth to warm will go away. (realclimate.org)
Most of the film lives up to its title, a very silly idea for a movie without much plot except to get the two idiots into comic situations. (qwipster.net)