OK, so after a bit of searching, it seems that it comes from the Latin word for "thousand", millia[1]. And it's apparently common in financial contexts.
So 1M is 1,000. 1MM is 1,000,000. 1MMM is 1,000,000,000 (though the former and latter are not as common). Still seems like a confusing way to abbreviate to me.
So 1M is 1,000. 1MM is 1,000,000. 1MMM is 1,000,000,000 (though the former and latter are not as common). Still seems like a confusing way to abbreviate to me.
[1]: http://en.wikipedia.org/wiki/Mile