thousand = k
million = m
M = a Roman numeral for 1000
billion (USA) = a cardinal number followed by 9 zeros (E.g.: 1,000,000,000)
billion (UK) = a cardinal number followed by 12 zeros (E.g.: 1,000,000,000,000)
M also means thousand, and has often been used so in various business and other writing. Besides the abbreviation mn., and the metric M, the construct mm is often used in technical business writing in North America to mean a million.
(Durma-se com um barulho desses!)