Sneak-a-poo is an unexpected poo. It's what happens when you go to the bathroom to pee, and then unexpectedly realize you have to poo too. It's like your poo is saying "peek-a-boo! I'm here!"
It usually only happens to girls, or to boys who sit down to pee.
It usually happens at an inconvenient time, like when you have a meeting in 5 minutes, or when you're at a friend's house for dinner and food has just been served, or when you're at your boyfriend's house in the middle of a movie and you ask him to pause it so you can quickly pee but then you're gone for 15 minutes. If you realize you have to poo in any of these situations or ones like it, you've been sneak-a-pooed.
As a noun:
Sorry I was gone for so long. I thought I just had to pee but it was really a sneak-a-poo.
As a verb:
Dude, sorry I made you wait for me outside the airport terminal bathroom after we deplaned and you were left standing there for 20 minutes and got so bored that you started riding the moving walkway for fun; I totally got sneak-a-pooed.