crackbrained, idiotic (adj)

insanely irresponsible

"an idiotic idea"

absurd, cockeyed, derisory, idiotic, laughable, ludicrous, nonsensical, preposterous, ridiculous (adj)

incongruous;inviting ridicule

"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"

imbecile, imbecilic, idiotic (adj)

having a mental age of three to seven years

idiotic (Adjective)

Pertaining to or resembling an idiot; afflicted with idiocy.

idiotic (Adjective)

Having the quality of idiocy; very foolish; stupid; nonsensical.