WordNet sense 1 (ludicrous, foolish):

cockamamie, goofy, sappy, silly, zany, cockamamy, whacky

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

foolish

WordNet sense 3 (informal or slang terms for mentally irregular):

balmy, batty, bonkers, buggy, cracked, crackers, daft, dotty, fruity, haywire, kooky, loco, loony, loopy, nuts, nutty, barmy, kookie, around the bend, bats, round the bend

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

insane