Monsieur means "My Lord" in French, and is now generally used as an honorific for all men, the equivalent to the English "Mister."
Under the French monarchy, Monsieur was an informal title given to the eldest living brother of the reigning King. Those who have been known by this title include:
Monsieur Zenith the Albino is an ambiguous villain created by writer Anthony Skene for his Sexton Blake series of detective pulp fiction.
The influence of Sherlock Holmes is evident, as Monsieur Zenith is himself a master of disguise and accomplished violinist, and there is a dash of Fantômas in him as well.