Sire is a respectful form of address for reigning kings in the United Kingdom and in Belgium. It has also been used in France, Italy, Germany, Portugal, Sweden and Spain. Historically Sire had a wider usage. During the Middle Ages, Sire was 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.
References
Sire Wikipedia(Text) CC BY-SA