The most offensive gesture in America is flipping someone off. Shooting the bird. Giving them "the finger." The gesture means screw you. Bug off. Or, to be more blunt, f*** you. The gesture involves sticking up your middle finger with your palm facing in. You can make the sign calmly, or angrily extend your arm toward the intended recipient, depending on how upset you are.
In the past, the gesture was a bit shocking to see. Not so much anymore. A news database search reported by NPR noted the phrase "giving the finger" was used three times more from 2000 to 2010 than it was between 1990 and 2000. And as anyone living in America knows, you'll see an impressive (dismaying?) assortment of people using it, from prominent politicians to movie stars [source: Weeks]. If this keeps up, "the finger" may no longer be seen as shocking and crude. Which likely means another gesture will take its place as most offensive.
"The finger," incidentally, is largely understood and used throughout the Western world, even though other countries might have their own preferred gestures to impart the same meaning [source: Language Trainers].