I answered this question 2 months ago, see link under sources:
"Fore" is another word for "ahead" (think of a ship's fore and aft). Yelling "fore" is simply a shorter way to yell "watch out ahead" (or "watch out before"). It allows golfers to be forewarned, in other words.
The British Golf Museum cites an 1881 reference to "fore" in a golf book, establishing that the term was already in use at that early date (the USGA suggests the term may have been in use as early as the 1700s). The museum also surmises that the term evolved from "forecaddie."
A forecaddie is a person who accompanies a group around the golf course, often going forward to be in a position to pinpoint the locations of the groups' shots. If a member of the group hit an errant shot, the thinking goes, they may have alerted the forecaddie by yelling out the term. It was eventually shorted to just "fore."
A popular theory is that the term has a military origin. In warfare of the 17th and 18th century (a time period when golf was really taking hold in Britain), infantry advanced in formation while artillery batteries fired from behind, over their heads. An artilleryman about to fire would yell "beware before," alerting nearby infantrymen to drop to the ground to avoid the shells screaming overhead.
So when golfers misfired and send their missiles - golf balls - screaming off target, "beware before" became shortened to "fore."
This is another term, however, whose exact origin can't be stated. It does originate, however, in the fact that "fore" means "ahead" and, used by a golfer, is a warning to those ahead.