“Thumbs down, signified “swords down,” which meant the losing gladiator was worth more to them alive and was to be spared to fight another day, not to be killed.

The belief that the ‘thumbs-up’ and ‘thumbs-down’ gestures gave approval or disapproval respectively entered the public consciousness with Jean-Léon Gérôme’s 1872 painting ‘Pollice Verso’. The ‘thumbs down’ gestures of the crowd in Gérôme’s popular picture were interpreted by the 19th century public as signs of disapproval. Actually, the artist probably never intended that, as ‘pollice verso’ just means turned thumb.”