Fore!
The traditional warning shout when a ball might be heading toward someone, always yell it, immediately, if there's any doubt.
The exact origin is disputed. The most plausible theory traces it to "forecaddie," a caddie sent ahead to spot where balls landed, golfers likely shortened their shout to that caddie into just "Fore!" A rival theory ties it to a military warning shout, though historians find little evidence connecting the two. Either way, the word was already in golf use by 1881.