What is the meaning of Mons?

It is possibly an abbreviation for "Monsieur" (Sir), although the standard one is "M.", not "Mr." or "Mons."

"Mon" means "My" (sing.), but the plural form is "Mes", so it does not seem to fit.

Mons, in the Latin, means Mount, as in Mons Olympus (Mount Olympus), and in Anatomy it means 'mound' referencing a localized mound of tissue such as the Mons Pubis