“Thumbs up” is a thumb signal, a common hand gesture achieved by a closed fist held with the thumb extended upward in approval or downward in disapproval.
The source of the thumb gesture is not certain but a number of origins have been proposed.
According to Anthony Corbeill, a classical studies professor who extensively researched the practice, “thumbs up” in ancient Rome was a signal to kill the gladiator while “a closed fist with a wraparound thumb” meant sparing him. The reason for such meaning is clear. If the audience wanted the victorious man to plunge in his sword, they mimed the act with their hands, their extended thumbs stabbing the air in encouragement. If they wanted to spare the defeated fighter, they made the opposite sign — hiding their thumbs away. In a vast arena like the Colosseum, the kill/no-kill signals would have to be strongly contrasting to be visible at all.
But, over time, the sight of an upraised thumb has come to symbolize harmony and kind feelings. Desmond Morris in Gestures: Their Origins and Distribution traces the practice back to a medieval custom used to seal business transactions. The old English saying was, “Here’s my thumb on it!” The two people involved each wetted a thumb and then extended it, held upwards, until the two raised thumbs came into contact with one another. It is easy to see how this custom could support the idea of holding out a raised thumb as a sign of friendly agreement or approval.
Also, there is something optimistic, positive, and dominant about upward movements, and something essentially pessimistic, negative, and subordinate about downward movements. This dichotomy pervades the whole of our language and our thinking, and it is obviously going to have an impact on our gestures as well.
It has been suggested that during the Middle Ages, “thumbs up” was a signal from English archers preparing for battle that all was well with their bow and they were ready to fight.
The Oxford English Dictionary cites the earliest written instance of “thumbs-up” (with a positive meaning) as being from Over the Top, a 1917 book written by Arthur Guy Empey. Empey was an American who served in the British armed forces during World War I. He wrote: “Thumbs up, a Tommy’s expression which means ‘everything is fine with me’.”
Popularization in the US is attributed to World War II pilots, who used the thumbs-up to communicate with ground crews, or aircraft carrier crews, prior to take-off. In scuba diving, the thumbs-up gesture is a specific diving signal given underwater, in which the diver indicates that he or she is about to stop his or her dive and ascend.
On the Internet, the thumbs up gesture is shown as an icon and is associated with the term “like”—which means approval. Hitchhikers in the West traditionally use a thumb gesture to solicit rides from oncoming vehicles.
However, its perceived meaning varies significantly from culture to culture. The sign has a pejorative meaning in parts of West Africa and some of the Middle Eastern countries including Iraq and Iran where the sign is equivalent to giving the middle finger.
And we certainly want to say, “Please help me,” not “up yours!”