I think they're just saying "partner in crime," which literally means someone you commit crimes with, but in slang means someone who has your back no matter wha

I think they're just saying "partner in crime," which literally means someone you commit crimes with, but in slang means someone who has your back no matter what (e.g. a spouse), but in this case they wanted to emphasize that while the two of them don't commit crimes, they do get into a bit of mischief together. I have no idea why it was made into an acronym.