Why do golfers yell fore?

Golfers yell fore when it appears others are in danger of being hit by their golf ball.

More about why golfers yell fore.

In modern golf the call of fore is used to alert others that they might be struck by a golf ball heading their way. Most often it will be due a stray shot missing it's target but it could also be if someone has wandered onto the player's fairway and is not aware of their shot.

The origins of the call are not certain but the most generally accepted view is that, because in the early days golf balls were expensive, golfers often had "Forecaddies" that would stand around about where the ball would  land. The Forecaddies would find the balls and thus limit the number of balls lost.

It's considered most likely that a golfer would shout to their Forecaddie to ensure their attention when a wayward shot was hit.  Over time the shout of 'forecaddie' being shortened simply to 'fore!