Why do golfers shout 'fore' (or 'four' or 'for' or 'foor') when they want to go through on the golf course?

Calling FORE is a warning call and means that a wayward shot has been hit and is somtetimes accompanied by a direction to alert golfers to duck eg. FORE LEFT would let golfers on the left side of a fairway know that there's a ball heading their way.