sir

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Used as an honorific before the given name or the full name of baronets and knights.
• noun Used as a form of polite address for a man.
• noun Used as a salutation in a letter.

from The Century Dictionary.

• To address as “sir.”
• To use the word sir.
• noun A master; lord; sovereign.
• noun A person of rank or importance; a personage; a gentleman.
• noun Master; mister: a respectful and formal title of address, used formerly to men of superior rank, position, or age, and now to men of equal rank, or without regard to rank, as a mere term of address, without etymological significance.
• noun Specifically— A title of honor prefixed to the Christian names of knights and baronets, and formerly applied also to those of higher rank, as the king; it was also prefixed occasionally to the title of rank itself: as, Sir King; Sir Knight; Sir Herald.
• noun (b ) Formerly, a title of a bachelor of arts; hence, a title given to a clergyman; also, a clergyman.
• noun A Persian measure of weight, equal to 16 miskals or ounces troy.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete A man of social authority and dignity; a lord; a master; a gentleman; -- in this sense usually spelled sire.
• noun A title prefixed to the Christian name of a knight or a baronet.
• noun An English rendering of the LAtin Dominus, the academical title of a bachelor of arts; -- formerly colloquially, and sometimes contemptuously, applied to the clergy.
• noun 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.
• noun See under Reverence, n.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A man of a higher rank or position.
• noun An address to a military superior of either sex.
• noun An address to any male, especially if his name or proper address is unknown.
• verb to address somebody using sir

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun term of address for a man
• noun a title used before the name of knight or baronet