I had written a post and sent out a few queries to people asking for information on how people with superfluous nipples were viewed in other cultures: witches, gods, devils, magicians, shamans, freaks... Even Google was no help. (Believe me, I've searched.)
Anyway, I got this email in reply. I'm not sure what part of Europe she's talking about, but based on her email address I'd guess Spain, Portugal, or Italy.
Regarding your question: According to Grandma, this belief goes back to Europe.
There was a very strong belief that people born with three nipples or more, were witches/warlocks. The belief was so strong, that when a baby was born, if it had more than two nipples the midwife would have to kill it!
If it was later discovered that the child had been permitted to live, the townspeople would go and hunt down the midwife and burn her alive. Mother and child would also suffer the same type of death.
Well, no wonder that some people are still reluctant to show their three nipples in public. Thank goodness times have changed. If Kate Hudson can walk around happy with her six toes, then we can wiggle our nipples at the world with pride.
And screw those angry townspeople. Pitchforks don't scare me.