Yoo-hoo is a call you use to get someone's attention when they are some way off, or when they haven't noticed you yet. You sing it out — the voice goes up and then down, like a little two-note tune — and you often wave at the same time. You use it across a car park, up a staircase, into a garden, or through an open door when you arrive at a friend's house: 'Yoo-hoo! Anyone home?' The feeling of it matters more than the meaning. It is friendly, cheerful, and soft — you would use it with a friend, a neighbour, a child, never to a stranger in a serious situation and never to a boss. It also carries an old-fashioned and slightly feminine flavour; in films and TV it is often a woman calling to someone she knows well, and people today often say it as a small joke, putting on a sing-song voice. Because of that joking use, it has a second life: you can say 'yoo-hoo?' with a rising tone at someone who is daydreaming and not listening to you, meaning 'hello, are you awake in there?' That one can sound teasing, so be a little careful. Write it with a hyphen. In the US there is also a chocolate drink called Yoo-hoo — a brand name, nothing to do with the call.