The vertical middle finger first appeared at the end of the Hundred Years War between Britain and France. The British archer caused the French army to suffer heavy losses. The French army vowed to cut the middle finger of the British army bow and arrow after defeating the British army.
As a result, the French army retreated and the British archers extended their right middle fingers to show off their middle fingers. Since then this insulting gesture quickly became popular in Western countries.
PS: The middle finger is raised in sign language to represent the older brother, which means that it will become the pillar of the family.