Dictionary:

kooky

  (kū'kē)

Characteristic of a kook; strange or crazy.

adjective

Deviating from the customary: bizarre, cranky, curious, eccentric, erratic, freakish, idiosyncratic, odd, outlandish, peculiar, quaint, queer, quirky, singular, strange, unnatural, unusual, weird. Slang screwball. British Slang rum, rummy². See usual/unusual.

The adjective has one meaning:

Meaning #1: informal or slang terms for mentally irregular
  Synonyms: balmy, barmy, bats, batty, bonkers, buggy, cracked, crackers, daft, dotty, fruity, haywire, kookie, loco, loony, loopy, nuts, nutty, round the bend, around the bend, wacky, whacky