During the great witch craze of about 1100 to 1700 (wow, trends sure lasted a long time back then) it was commonly believed (by the same kind of morons you might find today in, oh, Dover PA) that an extra nipple was the mark of the devil. Witches were supposed to nurse their familiars through the extra milk spigot.
It was widely believed that most witches sported a mark on their body which was placed there by the Devil. The Witches' mark, Devil's Mark, or Witches' teat was the seal of the Devil, given to witches upon initiation. This mark could be a scar, a mole, a birthmark, or superfluous nipple. The Devil's Mark was a nipple through which the witch nourished her or his familiar.
The popular saying "cold as a witch's tit" appears to refer to superfluous nipples too. I don't know about you, but mine is toasty warm right now. No doubt from bathing in the eternal fiery pits of hell.
More history of The Devil's Mark here and here.