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.
The others pretty much got it right. I'm almost 50 and at this age, feel strange calling my partner my "boyfriend". So, I came up with the term Partner in Mischief because we have lots of mischievous adventures together!
What do you mean? I haven't seen that expression anywhere and google wasn't helpful.