Iraqis are giving passing Americans the “thumbs up” sign, which the troops interpret as a symbol of support. But many veteran travelers insist that the gesture is a crass Middle Eastern insult. How should coalition forces take those skyward thumbs?

Depends on how media savvy those Iraqi bystanders may be. It’s true that “thumbs up” traditionally translates as the foulest of Iraqi insults—the most straightforward interpretation is “Up yours, pal!” The sign has a similarly pejorative meaning in parts of West Africa, Russia, Australia, Iran, Greece, and Sardinia, according to Roger E. Axtell’s book Gestures: The Do’s and Taboos of Body Language Around the World. So, it’s possible that the ostensibly cheering Iraqis are, in fact, silently voicing their displeasure.

But it’s also possible that they understand the Western meaning of upturned thumbs, an explanation that the Army’s Defense Language Institute subscribes to. According to a recent DLI manual on international gestures, after the first Gulf War “Middle Easterners of the Arabian Peninsula adopted this hand movement, along with the OK sign, as a symbol of cooperation toward freedom.” Iraqi civilians may have noted this shifting meaning, perhaps via TV reports.

How the thumbs up became an upbeat gesture in the first place is something of a mystery. Legend has it that the signal dates back to Roman gladiatorial contests. A beaten combatant could supposedly be saved from a death blow if the emperor gave the thumbs up; a thumbs down was tantamount to an execution order. Though a favorite of Hollywood “swords and sandals” epics, this explanation has been completely debunked in recent years. In 1997, University of Kansas classics professor Anthony Philip Corbeill concluded that the thumbs up actually meant “Kill him,” basing his assertion on a study of hundreds of ancient artworks. Instead, he wrote, a closed fist with a wraparound thumb was the indication for a gladiator’s life to be spared.

No one’s quite sure about where the positive American connotation comes from, though a good guess ascribes it to an English symbol of agreement. Desmond Morris’ Gestures: Their Origins and Distribution traces the practice back to a medieval custom used to seal business transactions. The two involved parties would lick their thumbs, hold them erect, then smush them together. Over time, the mere sight of an upraised thumb came to symbolize harmony and kind feelings.

The gesture’s popularization in America is generally attributed to the practices of World War II pilots, who used the thumbs up to communicate with ground crews prior to take-off. American GIs are reputed to have picked up on the thumb and spread it throughout Europe as they marched toward Berlin.

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Explainer thanks James Felt for asking the question.