WordWeb Online

1. Ludicrous, foolish
   "gave me a whacky reason for not going"
   - cockamamie [N. Amer], cockamamy [N. Amer], goofy, sappy, silly, wacky, zany
    
2. Informal or slang term for mentally irregular
   "it used to drive my husband whacky"
   - balmy [archaic], barmy, bats, batty, bonkers, buggy [N. Amer], cracked, crackers, daft, dotty, fruity, haywire, kooky, kookie, loco, loony, loopy, nuts, nutty, round the bend, around the bend, wacky, daft as a brush [Brit], wacko, round the twist [Brit]

Derived forms: whackiest, whackier

See also: daft, dumb, foolish, insane, stupid