Sire is an archaic respectful form of address to reigning kings in Europe. In French and other languages it is less archaic and relatively more current. In Belgium, the king is addressed as "Sire..." in both Dutch and French.
The words "sire" and "sir", as well as the French "(mon)sieur" and the Spanish "señor", share a common etymological origin, all ultimately being related to the Latin senior. The female equivalent form of address is dame or dam.