(1):
(n.) A title prefixed to the Christian name of a knight or a baronet.
(2):
(n.) A man of social authority and dignity; a lord; a master; a gentleman; - in this sense usually spelled sire.
(3):
(n.) A respectful title, used in addressing a man, without being prefixed to his name; - used especially in speaking to elders or superiors; sometimes, also, used in the way of emphatic formality.
(4):
(n.) An English rendering of the LAtin Dominus, the academical title of a bachelor of arts; - formerly colloquially, and sometimes contemptuously, applied to the clergy.