MM is an abbreviation for "million", short for "mille mille", or 1000 × 1000.
Encyclopaedia of pure mathematics, by Barlow, Peacock, et al., p. 426:
(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.