French artist Jean-Léon Gérôme produced a painting that was first exhibited in 1873 and became very popular in the form of engravings.
This painting showed a Roman emperor giving a ‘thumbs-down’ sign to a gladiator to kill his defeated opponent. Perhaps the legend arose from this painting, for as far as it is known, the signal for death in the gladiatorial arena was never ever indicated by the turning down of the emperor’s thumb.
In modern popular culture, it is assumed that a thumbs-down meant that the defeated gladiator be condemned to death while thumbs-up was the signal that he should be spared. One famous example is the Academy Award winning Gladiator(2000).
A 1693 translation of the Satires of Juvenal (AD 60-c.128) referred to ‘where with thumbs bent back they popularly kill’. The classical scholar John Mayor, in his 1853 edition of Juvenal’s work expained: ‘Those who wished the death of the conquered gladiator turned their thumbs towards their breasts, as a signal to his opponent to stab him to death; those who wished him to be spared turned their thumbs downwards. Why downwards? Because apparently that was the signal that the gladiator must drop his sword and spare the fallen gladiator’s life.
Before giving a judgement-the life-or-death signal to the victorious gladiator, it is said that Roman emperors would seek the spectators’ guidance.
The Latin Dictionary of Lewis and Short (1880) gives, under the entry for pollex (latin for thumb): ‘To close down the thumb (premere) was a sign of approbation; to extend it (vertere, convertere, pollex infestus) a sign of disapprobation.’