In other words, 70 or 80 years ago, Shanghai was still a concession, so there were many foreigners, especially the British, accounting for the vast majority! In order to maintain law and order in the Shanghai Concession, the British recruited many Sikhs from India to be public security police (then called patrol).
These Indians have dark skin. Because of their religious beliefs, they are all wrapped in red scarves. So Shanghainese originally called these Indian policemen "red-headed and black-charcoal".
However, civil servants in the British system (especially the police) have long been used to being called Mr. (see Jackie Chan's police series movies for details. Don't they all call Mr. A or Mr. Police? )。 In addition, Shanghainese have always been used to adding an "A" before monosyllabic words. So the Shanghainese shouted Sir Sam, and the Indian police called him "Red Head Sam".
A few months later, Shanghainese discovered that not everyone in India is a Sikh wearing a red scarf, and there are also many people of other religions in India who don't wear red scarves. Therefore, Shanghainese simply changed "Red Head Ah San" to "India Ah San", so this impolite nickname has been used to this day, and almost people in China are used to calling Indians "India Ah San".
There is also a highly credible sentence that has been included in the slang dictionary encyclopedia! At that time, people in Shanghai Concession often had to communicate with these Indian policemen, but China people's English was not good and Indian people's English was strange, so the "communication between police and people" in Shanghai at that time could be said to be a chicken talking with a duck, which was not smooth!
However, Shanghainese have actually found new pleasure in this kind of chicken-duck communication. Because they found that every time Indian police met glib Shanghainese, they always blushed and hesitated, and liked to stammer and repeat: "I said ... I said ...". As a result, Shanghainese pronounced the Indian police's classic stuttering sentence "I said" as "A San" in Shanghainese.
It is precisely because of the above two inexplicable reasons that the relationship between "Indian" and "Indian Sam" has been forcibly equated. Remember that Indians are really wronged! For no reason, I added the nickname "India Asan"!