The original meanings of "thumbs up" and "thumbs down"

Juvenal refers to the Roman custom of spectators’ voting on the fate of wounded gladiators with their thumbs. You may think a gladiator would appreciate the crowd’s “thumbs up” (verso pollice), but exactly the opposite is true. Where we give thumbs up as a sign of approval, it meant death to its Roman recipient; much to the crowd’s delight.

These men once were horn-blowers and attendants

At every municipal arena, known as trumpeters in every village.

Now they present their own spectacles, and, to win applause,

Kill whomever the mob gives the “thumbs up”.

Decimus Junius Juvenalis; a.k.a. Juvenal

(c. 55-140 A.D.), "Third Satire"

Thumbs down, signified “swords down,” which meant the loser was worth more to them alive than dead, and he was spared apparently so he could make up for his disgrace the next time he appeared in the arena. Keep this in mind the next time you give someone the “thumbs up” sign.

Our reverse interpretation of this custom apparently was the result of the work of the French artist Léon Gérôme who apparently understood the Latin verso ("turned") to mean "turned down", and therefore in his painting Pollice Verso (1873), he presents the death sentence with the thumbs-down gesture.

The painting became so popular that Gérôme’s mistake became the accepted interpretation and it is unlikely that it will ever be changed back to the meaning that it had with the Romans.

Scholars before Gérôme gave support to the view that “thumbs down” among the Romans, meant the hapless gladiator was to be spared, not slain.

The gesture meant "Throw your sword down". A 1601 translation of Pliny equates the gesture with "assent" or "favor", and John Dryden's 1693 version of Juvenal's Satires gives the thumb being bent back, not down, as the death signal.

Private investors spent enormous sums staging gladiatorial shows, despite attempts by various Roman governments to impose limits. Prisoners of war and slaves were sent to gladiatorial academies for training and then they were hired out for various public and private occasions.

The Arena Shows were often sponsored by various Roman governments and private businesses to keep the Roman mobs entertained

The shows sponsored by the governments and put on at public expense were initially held at the Circus Maximus, Rome's principal stadium for chariot racing. Later, the gladiators were moved into large amphitheaters, and posters that listed the combatants' names and successes from their past achievements. After a few preliminary spectacles, the gladiators got down to serious business.

Whenever a combatant was seriously wounded, the presiding judge, or referee, was called upon to determine whether the man should live or die, depending on how well he had put up a fight.

The judge usually based his decision on the desires expressed by the mobs in the stadium; whether they would cheer, applaud, and give the thumbs down if they liked the man, who was then carried away to be treated for his wounds.

If, on the other hand, they gave him the silent thumbs-up treatment, his opponent was given the signal to execute the mortal blow. The corpse was then dragged off like a dead animal.

—Based on information from It's Greek To Me! by Michael Macrone

and Fabulous Fallacies by Tad Tuleja