British English, MM means million.
Edit: you guys really will downvote anything, won't you?
I'm not from the UK though, and why they would eschew the simple-and-obvious m and b suffixes in favor of alternatives, especially when Europeans love to lecture us about how metric is so neat and intuitive.
Probably has something to do with the root words -- mil means 'thousand' in a lot of latin languages, and a thousand is M in roman numerals, and I heard somewhere that "MM" stands for "mil mil" or "million million" which, in the context of M equaling 1,000, woudl make sense.
Because not everyone is an American. Those suffixes are not used in other countries.
This is something that I didn't think would be this contentious. It's an old standard.
https://english.stackexchange.com/questions/181917/mixing-us...
It likely comes from SI prefixes. While there's not a standard base unit for currency, the fact they work for so many other units lends them towards easily inferred usage when applied to others.
1: http://www.npl.co.uk/reference/measurement-units/si-prefixes...