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Pollice Verso - Jean-Léon Gérôme (1872)
(upload.wikimedia.org)
Wander the gallery. Look at the art. Be polite. If you feel able please post some great art :)
Legend has it that, with the "thumbs up" being a modern expression, the Roman audience would extend the thumb in any direction to show "use the sword" and a clenched fist to say sheathe the sword/spare the opponent.