WordWeb Online

1. Informal or slang term for mentally irregular
   "it used to drive my husband kooky"
   - balmy [archaic], barmy, bats, batty, bonkers, buggy [N. Amer], cracked, crackers, daft, dotty, fruity, haywire, kookie, loco, loony, loopy, nuts, nutty, round the bend, around the bend, wacky, whacky, daft as a brush [Brit], wacko, round the twist [Brit]

Sounds like: cookie, cooky

Derived forms: kookier

See also: insane, kook [N. Amer]