A synonym for idiotic could be half-witted, stupid, or fatuous.

Other synonyms include:

crackbrained

insane

absurd,

cockeyed,

derisory,

idiotic,

laughable,

ludicrous,

nonsensical,

preposterous,

ridiculous

foolish

imbecile,

imbecilic

Preposterous, ridiculous, ludicrous, farcical, laughable, risible, idiotic, stupid, and foolish.

"Preposterous" means something that is absurd, ridiculous, or illogical to the point of being laughable or nonsensical.

Laughable = Pathetic, Pitiful, Absurd, Preposterous, Ludicrous, Mediocre, Rdiculous

ludicrous, ridiculous, nonsensical, irrational