A cuckoo is a type of long-tailed bird that's known for laying its eggs in the nests of other birds. In the summer, you're most likely to see cuckoos in various parts of Europe.
The famous German cuckoo clock is named for the cuckoo bird, and the cuckoo sound the clock makes each hour imitates the call of the cuckoo. To make this call is also to cuckoo. Informally, you might refer to eccentric or crazy people or situations as cuckoo. Though this informal meaning is credited to American English, the word has been used to mean "stupid person" since at least the 1580s.