Backwards

We're all familiar with the thumbs up gesture. It's pretty much universal meaning of "all ok" spans cultures and languages, and when asked, most people would say it's origin comes from the practice of the Roman emperor, or whoever was the big cheese at the time, signaling to a triumphant gladiator whether his defeated opponent should live or die.

Unfortunately, it seems the idea of thumbs up meaning "live" and thumbs down meaning "die" is completely the opposite from what actually happened, This is coming as such as surprise to many people that they are pointing to the Mandela Effect for an explanation regarding this mysterious switch. 

Crowd pleaser

It turns out the actual gesture, which the crowd performed, signaling their desire toward the emperor, was to wave the thumb in any direction for "kill", and hide the thumb, making it look just like a fist, for "die" the logic was the thumb represented the sword of the gladiator, so hiding it meant "put away", and otherwise meant "use". This would have been easier to see from afar. 

Pollice Verso

The 1872 painting from Jean-Léon Gérôme, Pollice Verso, shows a Gladiator fight with the crowd using the thumb gesture. In fact, Ridley Scott has quoted this as the inspiration behind a similar scene in his 2000 Gladiator movie. This wasn't the first time it was seen in the movies however, and examples can be traced back to the silent era.

Modern use

The thumbs up gesture meaning "all's well" might have a more recent origin. In World War 2, US pilots used it out of necessity in noisy environments, such as pre-flight cockpit checks. There's even evidence this was taken from a similar use by the Chinese.

In any case, with so many popular references now, ranging from Fonzies double thumbs up gesture, to emoticons in twitter, smartphones and electronic messaging systems, it seems this is here to stay no matter what the Romans used it for in their arenas.