1. the tip on a pile of poop
2. opposite of the icing on a cake, poop-nipple describes exacerbation on top of an already shitty situation. It is similar to the salt in "rubbing salt on the wound" and the kick in "kick them while they are down".
3. an unsympathetic person.
1. The google poop emoticon, ~@~, has a poop-nipple on top.
2. the break-up is the poop-nipple of an already terrible week.
3. that jerk is being a poop-nipple.