"Hey! Poo Poo"

The other day I was stopped at a light. It was nice weather so most people had their car windows down. Suddenly, from the vehicle behind me, someone yelled.
"Poo Poo! Hey, Poo Poo!"
I looked into the rear-view mirror and saw a black guy hanging out the passenger-side window of the car behind me, yelling at a girl in the next car. She looked over and smiled, obviously happy at being called "Poo Poo."
Poo Poo? I thought. Her name is Poo Poo?
I continued to watch these two exchange words and carry on an animated conversation. They were obviously close and had probably known each other since childhood.
I can't image her acquiring that nickname as an adult.From my angle, I couldn't tell what Poo Poo looked like so I can't share whether or not she deserved the nickname from any physical traits. Like a hairstyle that looked like shit. Or a pock-marked face from years of debilitating acne.
Poo Poo has to be a nickname she got stuck with as a kid. Right?
What's really sad is that as a twenty- or thirty-something year old woman, she still answers to it. Somebody says Poo Poo and she looks up, smiles, and says something cordial back. This can only mean she finds the name endearing.
Poo Poo?
Now here's the real kicker. This is what bugs me the most and keeps me awake at night. What in the hell did she do in the first place to get branded Poo Poo?
Think about it.
Let your imagination run wild.
Get back to me.