Monsieur is a simplified form of ... Monseigneur
Isn't that classy? Monsieur is a contraction of mon and seigneur, i.e. monseigneur, but usage and the desire to move faster deformed it into "monsor" (attested as early as the 13th century) and then into monsieur. Originally, it was a way of greeting someone with deference, to show respect. You'd say "monsieur" to someone important. From the seventeenth century onwards, the custom spread throughout society.
How do you spell "Monsieur"?
Monsieur is abbreviated to M.
But be careful: when you're addressing someone directly, you need to spell out Mr. in full. For example, in a letter, on an envelope or in an e-mail.
