https://news.ycombinator.com/item?id=45240304#45241147

  To represent one million in finance, the abbreviation “MM” is widely used. This notation originates from “mille mille,” meaning “thousand thousands” in Latin, equating to one million. This clarity makes “MM” a preferred choice in financial statements and reports.

I feel like the mille notation might have been usable before trillions started getting thrown around. Nowadays, k, M, B, and T seem like clearer suffixes than M, MM, MMM, and MMMM.