MM is an abbreviation for "million", short for "mille mille", or 1000 × 1000.

[_Encyclopaedia of pure mathematics_, by Barlow, Peacock, _et al._, p. 426][1]:

> (99.) It has been from a very early period the custom of writers on Arithmetic to separate numbers into periods of three and six, as the numeration in most European languages must proceed by thousands and millions; these periods are called _membres_ by Stevinus, amongst whose _definitions_ we find the following: …. Instead of _million_, he says, _mille mille_; for a _thousand millions_, he uses _mille mille mille_; and for a _billion_, _mille mille mille mille_, and so on for higher numbers.

  [1]: https://books.google.com/books?id=3fIUAQAAMAAJ&pg=PA426&vq=mille