(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.