What is "MM"? Millimeters? I'm joking of course, but I don't understand what "MM" means in a money context. I know that $1K is one thousand dollars, $1M is o

What is "MM"? Millimeters? I'm joking of course, but I don't understand what "MM" means in a money context. I know that $1K is one thousand dollars, $1M is one million dollars, $1B is one billion dollars, $1T is one trillion dollars, and gosh, by now, we're talking some serious coin... but "MM"? Even if MM were Roman Numerals, one would have to assume an implied multiplication.