The letter M is derived from the Phoenician Mem, via the Greek Mu (Μ, μ). Semitic Mem probably originally pictured water. It is known that Semitic people working in Egypt c. 2000 BC borrowed a hieroglyph for "water" that was first used for an alveolar nasal (/n/), because of the Egyptian word for water, n-t. This same symbol became used for /m/ in Semitic, because the word for water began with that sound.
http://en.wikipedia.org/wiki/M
For other uses, see M (disambiguation).