The title of the film is a line from a nursery rhyme,
Vintery, mintery, cutery, corn,
Apple seed and apple thorn,
Wire, briar, limber lock
Three geese in a flock
One flew East
One flew West
And one flew over the cuckoo's nest
Chief Bromden's (The writer of the original book) grandmother sang this song to him when he was young, and they had a game about it. A playful name for a mental asylum is a "cuckoo's nest", a mentally unstable person can be referred to as "cuckoo". To "fly over a cuckoo's nest" is to go too far, to get yourself in trouble. Though this can refer to the character of McMurphy being too much of a free spirit and eventually angering Nurse Ratched so much that he receives a lobotomy as result, it can also refer to the ending, where two characters died, and Chief Bromden escaped the Asylum or "Flew Over the Cuckoos Nest". It is also known that Cuckoos lay their eggs in other birds' nests, and do not have nests of their own. The Cuckoo, upon hatching, throws the other birds out of the nest out of instinct.