What is Cuckoo ?
Cuckoo is (noun) a bird which has a cry that sounds like its name and which lays its eggs in other birds’ nests When you hear the first cuckoo you know that winter is over. The cuckoo lays its eggs in the nests of other birds. a cuckoo in the nest someone who comes into a family or organisation and gradually takes control(adj) mad (informal.) They must be cuckoo! Synonym eccentric. Antonym ordinary
source: Easier English, Student Dictionary Upper Intermediate Level