What is Mundum? I'd only ever heard that word from an evil legalist (I say "evil" as a way to remind my brain to avoid talking to her, she's said before that she thinks that, yes, all of my extended family deserve to be murdered and yet still she's convincing sometimes) I knew on Facebook, you seem much nicer than she. I assumed that it was from Augustine, that was what my google searches turned up, but she didn't recognize it from Augustine. Is it from some anime?

In this case, it’s a reference to The Northern Caves, a brilliant little piece of fiction by @nostalgebraist.  One character within that work uses “Mundum” to mean “the completely objective orthogonal-to-human-welfare force of moral demand in the universe” (which he believes to be very real).