Absurd. Also:
asinine, balmy, brainless, childish, crazy, dippy, dizzy, empty, empty-headed, fatuous, featherbrained, flighty, foolhardy, foolish, frivolous, giddy, harebrained, idiotic, ignorant, illogical, immature, imprudent, inane, inappropriate, inconsistent, irrational, irresponsible, ludicrous, meaningless, muddle-headed, nitwitted, nonsensical, pointless, preposterous, puerile, ridiculous, senseless, simple, simple-minded, stupid, unintelligent, unreasonable, unwise, vacuous, wacked out, witless.