A male nipple. The logic behind mipple is that a male breast is not called a boob but a moob, which is short for man boob. Thus a the shortened form on man nipple is mipple.
The physical characteristic affecting various men in which the nipples are unreasonably large and erectly protrude past the point of normalcy.
Mipples lead to the debilitating condition where a man can be "mippling", which causes chafing or bleeding when exposed to particularly harsh fabrics or natural fibers like wool, as well as embarrassment when wearing thin t-shirts in public places.
JesusBetty, look at the mipples on that guy! They look like two thimbles on an ironing board."