I always thought it was short for "million monthly" so when you see something like "we have 10MM users" it would be 10 million monthly active users (i.e. 10 million users who have been active in the past month). I have no idea what it means here.
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.
