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It's million, derived from M, Roman numeral for one thousand, one thousand one thousands is one million.

I can't find any source for this, but it seems more likely MM is an abbreviation of "mille mille."

MM in Roman numerals would just be 2000. 1 million would be written as M with a bar over it. [1]

I think it's more directly derived from the French 'Mille Mille ', or 'Thousand Thousand ', used in finance.