Kids believed giving someone the middle finger was universally understood as the ultimate rude gesture, and that it was especially offensive or meant something different in other countries like China, Japan, or the Middle East.
While the middle finger is offensive in many Western countries, it's relatively meaningless in many other cultures. In China, it's not traditionally a significant insult. Other cultures have far more powerful gestures, the V-sign is offensive in the UK and Australia, thumbs up is insulting in some Middle Eastern countries, and the left hand alone is taboo in parts of Africa and Asia. Rudeness is entirely culturally dependent.