WordNet sense 1 (incongruous;inviting ridicule):

absurd, cockeyed, derisory, laughable, ludicrous, nonsensical, preposterous, ridiculous

WordNet sense 2 (devoid of good sense or judgment):

foolish

WordNet sense 3 (insanely irresponsible):

crackbrained

WordNet sense 4 (afflicted with or characteristic of mental derangement):

insane

WordNet sense 5 (having a mental age of three to seven years):

imbecile, imbecilic

WordNet sense 6 (lacking intelligence):

unintelligent