Sire was a respectful form of address for reigning kings in Europe. It was used in Belgium, France, Italy, Germany, Sweden, Spain, and the United Kingdom. Sire is generally used to address a superior, a person of importance or in a position of authority, or the nobility in general.
The words "sire" and "sir", as well as the French "(mon)sieur", share a common etymological origin, all ultimately being related to the Latin senior. The female equivalent form of address is dame or dam.