The thumb up is the universal sign for “well done,” “nice job,” and “I approve.”
Except — it isn’t universal at all.
In the United States and many other nations, yes, it’s positive, ranging in meaning to a denotative “OK” all the way to the more colloquial “awesome.” But in parts of the Middle East and Asia — Iran, Bangladesh, Thailand, perhaps Iraq (opinions differ) — it’s anything but. Giving someone a thumbs up there is akin to “giving them the finger” here in the U.S. Probably not a good idea.
In fact, the “thumbs up” sign isn’t, historically, all that positive. As Slate reported a few years back, the original meaning was most likely an order dating back to the Holy Roman Empire: “Kill him.” The more modern, Western, positive connotation is likely medieval (and unsanitary) in nature, an off-shoot of an English custom. Upon agreeing to a deal, the two sides would seal it by licking own thumbs, holding them upright, and smushing them together.
Today, we’ve gotten rid of the licking and smushing, thankfully, and gone with the gesture pictured above.
From the Archives: Fatal Error: Briefly discusses the role of the thumbs up (and thumbs down) in gladiator battles.
Related: This. Why it exists is anyone’s guess.