Hand signal for "out".

Many, many years ago the fist and thumb was used, before my time. Many untrained umpires still do it. I started umpiring in 1975 and the closed fist was the accepted mechanic. What learned was a basically a hammered fist, although truthfully it was probably between the old thumb and the present hammer. As the training developed the hammer has developed into a step a bang out, happens on close plays.

Hand signals started when the center field for the Detroit Tigers when Ty Cobb played was def and could not understand what the umpire was saying. I would start there.

there are a number origins of hand signals that get the most credence....

Cy Rigler is believed to have created signals for balls and strikes while working in the minor leagues...

The November 6, 1886 issue of The Sporting News, said the deaf pitcher Ed Dundon is credited as using hand signals while umpiring a game in Mobile, Alabama....

On his Hall of fame plaque, the Umpire great Bill Klem is credited with bringing the signals to the major leagues so that the crowds and fans could hear the calls...(no PA system back then)....

Then the case of William "Dummy" Hoy....who had his third base coach signal to him whether of not the pitch was a ball or strike when he batted.....Hoy was deaf...and a centerfielder.....but never played with Cobb....

Signals and mechanics are an ever evolving thing......