Preposterous is a good long word including the idea of stupid in its definition. Preposterous adds an element of the absurd to its meaning, lending more outrageousness to its definition than simply a lack of intelligence. When an object is preposterous, it is ridiculous or laughable, according to Webster.
Boneheaded is more light-hearted than stupid, but a boneheaded idea, for example, can be a stupid idea. Use boneheaded or call someone a bonehead as a form of slang. Beware, though, because boneheaded is about as derogatory as stupid and name-calling isn't nice.
Fatuous is a fancy way to refer to someone or something as stupid. Fatuous sounds less like a hiss than stupid. When someone is fatuous, her stupidity is silly or inane, according to Webster. Add to that the idea that the object is not just stupid, but "complacently stupid" and you have flung a full-fledged insult.
Imbecile or imbecilic may sting even more than stupid stings. In addition to foolish and moronic, imbecilic carries the stigma of mental retardation. Best advice: don't call people imbeciles or actions imbecilic.