whacky(adjective) adjective1. ludicrous, foolishSimilar word(s): foolish, goofy, silly, wacky, zany2. informal or slang terms for mentally irregularSimilar word(s): insane, balmy, barmy, bats, batty, bonkers, buggy, cracked, crackers, daft, dotty, fruity, haywire, kookie, kooky, loony, loopy, nuts, nutty, wacky