In
politically-correct parlance, an "
auxiliary" or
third nipple.
Auxiliary (third) nipples are often small and appear to be moles that swell when touched and do not change colour in the sun.
Anne Boleyn had one, as do roughly 1/3 of all humans. They're kind of like inflamed appendixes or wisdom teeth: some of us have, some of us don't.
Colder than a witch's tit is an old English gripe about inclement weather.