Insanely irresponsible:
— An idiotic idea.

synonym: crackbrained.

Roget 450a: unendowed with reason, void of reason; thoughtless; vegetative; moronic [Sarc.], idiotic [Sarc.], brainless [Sarc.].   

Polish: durnowaty, kretyński, debilny, zidiociały, idiotyczny

So unreasonable as to invite derision.

synonyms: absurd, cockeyed, derisory, laughable, ludicrous, nonsensical, preposterous, ridiculous.

Roget 499: unintelligent [Applied to persons], unintellectual, unreasoning; mindless, witless, reasoningless^†, brainless; halfbaked; having no head etc. 498; not bright etc. 498; inapprehensible^†.    weak headed, addle headed, puzzle headed, blunder headed, muddle headed, muddy headed, pig headed, beetle headed, buffle headed^†, chuckle headed, mutton headed, maggoty headed, grossheaded^†; beef headed, fat witted, fat-headed.    weak-minded, feeble-minded; dull minded, shallow minded, lack-brained; rattle-brained, rattle headed; half witted, lean witted, short witted, dull witted, blunt-witted, shallow-pated^†, clod-pated^†, addle-pated^†, addle-brained^†; dim-sighted, short-sighted; thick-skulled; weak in the upper story.    shallow, borne, weak, wanting, soft, sappy, spoony; dull, dull as a beetle; stupid, heavy, insulse^†, obtuse, blunt, stolid, doltish; asinine; inapt etc. 699; prosaic etc. 843; hebetudinous^†.    childish, child-like; infantine^†, infantile, babyish, babish^†; puerile, anile; simple etc. (credulous) 486; old-womanish.    fatuous, idiotic, imbecile, driveling; blatant, babbling; vacant; sottish; bewildered etc. 475.    blockish^†, unteachable Boeotian, Boeotic; bovine; ungifted, undiscerning^†, unenlightened, unwise, unphilosophical^†; apish; simious^†.    foolish, silly, senseless, irrational, insensate, nonsensical, inept; maudlin.    narrow-minded etc. 481; bigoted etc. (obstinate) 606; giddy etc. (thoughtless) 458; rash etc. 863; eccentric etc. (crazed) 503.    [Applied to actions] foolish, unwise, injudicious, improper, unreasonable, without reason, ridiculous, absurd, idiotic, silly, stupid, asinine; ill-imagined, ill-advised, ill-judged, ill-devised; mal entendu [Fr.]; inconsistent, irrational, unphilosophical^†; extravagant etc. (nonsensical) 497; sleeveless, idle; pointless, useless etc. 645; inexpedient etc. 647; frivolous etc. (trivial) 643.    ... show more

Having a mental age of three to seven years.

synonyms: imbecile, imbecilic.