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.