Nipple. It's just a fun word used to describe 'protrusion'.
Something that 'pokes' or 'sticks' out. A 'nipple' was a nipple
before it was a body part. I believe a woman's 'nipple' have been
called many things before people got used to the fact it was just
an average body part all humans have (men included). That said, to
demystify the word "nipple" & bring it's truth to light, look
around and see just how many things actually have a 'protrusion' of
some sort and consider it might have been called a nipple before it
was named something else. Like, an "outty belly button" ...isn't
that just a stomach nipple?