Marriage Proposal Index. A simple statistic ranking your ability or plausibility of getting married. You get points for all sorts of things, based on your culture. In America, its probably mostly due to money and/or fame. In other places, social status. Sometimes based on profession (doctors can get married easily).
"Man, Denzel's MPI is so high, he doesn't have to worry about anything"
Simple coding language commonly used on MUCKs (see second definition of MUCK). It is an acronym, standing for "Message Parsing Interpreter."
He created a hotel on a MUCK, coding it entirely in MPI.