I remember as a kid, during swimming class, noticing one of my classmates with three nipples. He was the hit of the class that day, a one day hit wonder until someone mentioned that extra nipple was the mark of the Devil.
The Devil's mark, sometimes called a Witches' mark comes in two different forms, said to be natural and unnatural. Both were recognized as definite proof by authorities during the middle ages of a witch. Both marks would not bleed if pricked with a needle.
The first is a mark supposedly made by the Devil himself, to identify his flock, either by his teeth or his claws. Many people today believe they were really tattoos, a blueish, reddish spot much like a flea bite, in the shape of a hare's foot, or the foot of a rat or spider, deemed unnatural but superior to the other form, an inferior Third Teat. Definite proof because Lucifer made the mark, there was no other explanation.
The Third Teat
This form of a Devil's mark, the third teat was deemed natural and comes at child birth, and was also considered proof of a person being a witch in the middle ages. We know the Devil wasn't involved and really that they are natural not unnatural. The darker the blue the larger the spot, and is usually situated on the chest. In actuality it is the Supernumerary Nipple or Polymastia breast.
Since it didn't bleed when pricked, it was easily cut out to avoid any suspicion by people suspected of being a witch. It was said they secreted milk, which Familiars would suckle but as a rule they are functionless.
During the Great Wars I & II, the military had an opportunity to examine millions of men and women's chests and found a third teat to be quite a common occurrence.
If a third teat was discovered on a person accused of being a witch back in medieval times, it would be grounds for immediate execution. You can understand why someone would want to keep it a secret, back then, if one had one. I would!