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

[Middle English, variant of sire, sire. See sire.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.