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M does happen to be the Roman numeral for “thousand” — as seen in the credits of old movies. But let’s not go there.

k = thousand, M = million, G = billion

There is actually a standard for this. I'm fine with MM, it's confusing to me but not ambiguous, just please don't reassign existing prefixes...

This is a false-cognate in MM.

MM means millions, plural of M which means million.

Source? Every source I've seen says that MM is derived from the Roman numeral M, meaning thousand.

Wouldn't that make MM 2,000, the same way II is 2 and XX is 20?

It would be if it was interpreted as an actual Roman number. But in this case it's treated as M x M.

The actual Roman version of a million is an M with a bar over it, where the bar means x1000. But that's not an ordinary character, so wouldn't work for this purpose.