Thumbs down on the thumbs-up sign when you're traveling. This seemingly-innocuous gesture that means, "Great! I like it! All right!" in the U.S. has pretty awful meanings elsewhere. In the Middle East, for example, it means up your butt, fella. (Or worse.) Many, if not most, Latin Americans find it offensive, as do citizens of West Africa, Greece, Russia, Sardinia, the south of Italy, Australia, the Philippines and many Islamic nations. Phew! That's a lot of thumbs-up haters.
The gesture may have been popularized during World War II, when American pilots flashed the sign to their grounds crews to indicate they were good to go. But scholars believe it actually originated in ancient Rome when crowds used the "thumbs-up" sign to mean a gladiator should be speared or hid their thumbs if he should be spared. (Notice the negative meaning?) If you simply can't stop using this sign, know that you'll be all right in Germany and certain areas of Japan, where the thumbs-up sign simply indicates the number one [source: Koerner].