Adjectives

(s)crackbrained, idioticinsanely irresponsible“an idiotic idea”
(s)absurd, cockeyed, derisory, idiotic, laughable, ludicrous, nonsensical, preposterous, ridiculousso unreasonable as to invite derision“the absurd excuse that the dog ate his homework”, “that's a cockeyed idea”, “ask a nonsensical question and get a nonsensical answer”, “a contribution so small as to be laughable”, “it is ludicrous to call a cottage a mansion”, “a preposterous attempt to turn back the pages of history”, “her conceited assumption of universal interest in her rather dull children was ridiculous”
(s)imbecile, imbecilic, idiotichaving a mental age of three to seven years