Fore!
"Fore!" is the warning shout when a ball is flying towards other people.
"Fore" is the loud warning shout of golf. You shout it as soon as your ball could fly towards other players, spectators or course staff. The shout gives those in danger seconds to duck or turn away. It is not a courtesy but a duty: a golf ball reaches speeds that can cause serious injuries. Its origin is not entirely clear — it probably comes from "forecaddie", the ball spotters who ran ahead in early rounds.
In practice you shout immediately and as loudly as possible. Do not wait to see whether the ball really hits. An example: your tee shot curves as a slice onto the neighbouring hole, where a group is just putting. You shout "Fore", ideally with a direction such as "Fore right". The others duck, the ball lands two metres away from them. Without the shout the ball could have hit someone on the head. Long balls on your own hole also count, for example when the group in front of you is still within range.
One misunderstanding is the shame: some people do not shout because the missed shot embarrasses them. That is dangerous and unsporting. Always shout, even if the ball ends up reaching nobody. After the shout you briefly check that everything is okay and play on. More about etiquette on the course can be found in the FAQs and under the topic of pace of play.
FAQ
When exactly do I have to shout "Fore"?
As soon as your ball could fly towards other people — immediately and as loudly as possible, without waiting to see whether it really hits. This applies to mishits onto neighbouring holes just as much as to long balls on your own hole when the group in front of you is still within range. Ideally you add a direction such as "Fore right".
What do I do if my ball hits someone despite the warning shout?
Go over immediately and find out whether the person is injured — if in doubt notify the club office or the emergency services, because a golf ball can cause serious injuries. Apologise in a matter-of-fact way and exchange contact details if needed. The warning shout does not release you from responsibility, but it is the most important thing you can do in a fraction of a second.
What do I do when I hear "Fore" myself?
Do not turn around and look — duck immediately, protect your head with your arms and turn away. Only once the ball has landed do you straighten up again. You should explain this reflex reaction to beginners in your flight before the first round.