1960s: probably from cuckoo.

• cuckoo from Middle English:

  The cuckoo is one of those birds whose name echoes the sound of its distinctive call—other examples are curlew (Late Middle English), hoopoe (mid 17th century), kittiwake (mid 17th century), and peewit [E16th]. You can describe an unwelcome intruder in a place or situation as a cuckoo in the nest. This comes from the cuckoo's habit of laying her eggs to be raised in another bird's nest. Cuckold (Old English), referring to the husband of an unfaithful wife, also derives from cucu, and plays on the same cuckoo-in-the-nest idea, although it is not actually the husband who is being the ‘cuckoo’. The reason that a silly or mad person is described as a cuckoo, or is said to have gone cuckoo, is probably that the bird's monotonously repeated call suggests simple-mindedness. Kook, ‘an eccentric person’, is short for cuckoo. It was first recorded in the 1920s but only really became common in the late 1950s. See also cloud, coccyx