Before "k" was adopted as the colloquial way of writing a thousand (i.e. $35k to mean $35,000), it was common to use "M" instead; "M" being the Roman Numeral for 1,000. As a result, "M" simply became shorthand for adding three zeros and thus "MM" became the shorthand for adding six zeros. The term “grand” has been in use as an American slang from the early 1900s, presumably from the phrase “a...