frivolous, inconsequential, inconsiderable, insignificant, little, minor, negligible, slight, trifling, trivial, unimportant

absurd, asinine, balmy, brainless, crackpot, crazy, cuckoo, daffy, daft, dotty, empty-headed, fatuous, foolish, half-witted, harebrained, insane, jerky, kooky (also kookie), loony (also looney), lunatic, mad, mindless, nonsensical, nutty, preposterous, sappy, silly, simpleminded, stupid, tomfool, unintelligent, unwise, wacky (also whacky), weak-minded, witless, zany

irrational, unreasonable

aimless, haphazard, purposeless