Why do people call/ consider the middle finger the cursing middle finger the cursing?
Where did that concept originate from?
According to anthropologist Desmond Morris, the gesture probably came to the United States via Italian immigrants. The first documented appearance of the finger in the United States was in 1886 when Old Hoss Radbourn, a baseball pitcher for the Boston Beaneaters, was photographed giving it to a member of the rival New York Giants.
Thaat being said, there is a long history of using gestures as obscenities. In the late Renaissance and in the Early Modern Period of Europe, biting your thumb towards someone was considered similar to giving them "the finger" in modern times. Knights of the Medieval period would raise their lances in what seems to be a similar fashion.
Short answer: We don't quite know, but the history of using physical gestures as insults is quite long.