KOO-koo! Click below to hear this.
Yoohoo! Peek-a-boo! Knock, knock!
No, this is not how you call someone crazy. It is, however, the song of the cuckoo bird, and has a bunch of other uses as well.
In general, this is the French way of calling attention. You may stop in an open doorway and call Coucou! as an English-speaker might call Yoohoo! (Is anybody there?)
Or you can poke your head in the door of someone’s office, see your colleague sitting there working, and call Coucou! as an English-speaker might call Knock, knock! (Mind if I come in?) No, it is not the start of a knock-knock joke.
Or it can be a game you play with babies, but the rules are a little different than the peek-a-boo rules. Americans cover their face and say, Where’s Gram? And then you pop open your hands and say, Here she is!
French mamas cover their face and say Coucou! then open their hands and exclaim Et voilà! (There he/she/it is!) Et voilà! can also mean presto!, so you can exclaim it as you produce the long-awaited solution or an unexpected surprise. Or it can just mean the end of a story or a discussion, said with a shrug, as if to say, Well, there it is. What more can I say?