sir

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Used as an honorific before the given name or the full name of baronets and knights.
2. n. Used as a form of polite address for a man: Don't forget your hat, sir.
3. n. Used as a salutation in a letter: Dear Sir or Madam.

Century Dictionary and Cyclopedia

1. n. A master; lord; sovereign. The use of sir in this and the next sense is derived in part, if not wholly, from its use in address (def. 3); the regular form for these senses is sire. (See sire.) The Middle English forms cannot be discriminated in the plural.
2. n. A person of rank or importance; a personage; a gentleman.
3. n. 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. In emphatic assertions, threats, or reproaches the word takes meaning from the tone in which it is uttered. It was used sometimes formerly, and is still dialectally, in addressing women.
4. n. 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.
5. n. (b ) Formerly, a title of a bachelor of arts; hence, a title given to a clergyman; also, a clergyman.
6. To address as “sir.”
7. To use the word sir.
8. n. A Persian measure of weight, equal to 16 miskals or ounces troy.

Wiktionary

1. n. A man of a higher rank or position.
2. n. An address to a military superior of either sex.
3. n. An address to any male, especially if his name or proper address is unknown.
4. v. to address somebody using sir

GNU Webster's 1913

1. n. obsolete A man of social authority and dignity; a lord; a master; a gentleman; -- in this sense usually spelled sire.
2. n. A title prefixed to the Christian name of a knight or a baronet.
3. 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.
4. 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.

WordNet 3.0

1. n. term of address for a man
2. n. a title used before the name of knight or baronet