The exact origins of baseball umpires' use of hand signals is not exactly a clear matter of historical record, though it seems that umpires have been using hand signals since at least the late 1800s. There are several theories that explain the potential origins of this practice.
Consensus on who invented hand signals, when and why seems to be limited, with different baseball experts seeming to have different opinions. It has been suggested that deaf player William "Dummy" Hoy popularized the use of hand signals in the late 1880s, but newspaper records indicate that signals were used before Hoy's time in the big leagues. The Baseball Hall of Fame at Cooperstown attributes umpire hand signals to Bill Klem, a late 19th century umpire, but a 2010 documentary, "Signs of the Times," disputes this story.