Sen. Edward M. Kennedy gives a thumbs-up as he leaves Massachusetts General Hospital on Wednesday, May 21 2008.

Seems everyone these days is giving the thumbs-up, no matter the circumstances.

Senator Edward Kennedy gave a big thumbs-up as he left the hospital Wednesday, facing a new outlook on life with a potentially deadly brain tumor. Hillary did it just this week at a we're-not-mathematically-defeated-yet campaign rally in Florida. John McCain can't not do it.

The gesture, for better or worse, has long breathed life or death into major events.

Death to gladiators

The thumbs-up gesture has its roots in ancient Rome, where gladiators would literally live or die by it. Pollice verso is the Latin term for the gesture, meaning "with a turned thumb."

"It was a hand gesture that was used by the crowd to say if the gladiator should live or die after a fight," explains Lisa Slattery Rashotte, a sociologist at the University of North Carolina, Charlotte.

The last thing a sweaty sportsman wanted was a thumbs-up. While in modern times it has a positive meaning, back then it meant "get him out of here," or death, while a concealed thumb (considered thumbs-down) meant the gladiator lived.

War ready

During World War II, American pilots gave the thumbs-up a new spin.

"The most well-documented origin of it in the U.S. is from World War II," Rashotte told LiveScience. "Pilots would use a thumbs-up to indicate they were ready to go up. That's how they would indicate to the crew that they were ready, that everything was good."

The hand motion spread across Europe and even parts of South America during the war.

While many cheerful gestures have fallen out of popularity, such as the increasingly tired high-five and its really dated predecessor, gimme-five, the thumbs-up has kept a firm grip on cultures.

The media is one reason for the gesture's sticking power. "With the thumbs-up and the pilots, you saw that image a lot. It was a positive image from a war situation that could be brought back home and put in the papers," Rashotte said.

The thumbs-up took a small hit in the 1950s and 1960s as anti-war sentiments took center stage and the peace sign became the more popular positive gesture.

The Fonz!

Rashotte said the sitcom "Happy Days," which first aired in 1974, likely revived the thumbs-up. The show's too-cool character Fonzie (Arthur Fonzarelli, played by Henry Winkler), often flipped both thumbs-up while flashing a dashing grin and delivering a gravelly "aaaaaaaay."

With its widespread usage in the United States, the "everything is good" sign might be here to stay.

"People in the U.S. know that thumbs-up means 'good job' or 'I agree' or 'things are good.' In our culture, it seems to be one of the more fundamental kinds of nonverbal communications," Rashotte said.

In other regions, the thumbs-up sign is also popular, though it can have various meanings. In Iran and Greece, it can be the equivalent of "flipping someone off," Rashotte said. In India, the meaning changes depending on whether the gesture is accompanied by certain words or other hand movements, she added.

Editor's Note: Several readers questioned whether thumbs-up really meant death to Roman gladiators. While there is debate, Rashotte said in a follow-up telephone interview that scientists generally agree thumbs-up signaled death. As for the movie "Gladiators," the conventional usage of thumbs-up was portrayed, likely because that's what modern-day moviegoers would understand since the gesture wasn't explained in the movie, she suggested. "There is some debate about it, but I think it's pretty well-accepted among academic communities that thumb-up [means] put to death," Rashotte said.