a respectful or formal term of address used to a man: No, sir.
(initial capital letter) the distinctive title of a knight or baronet: Sir Walter Scott.
(initial capital letter) a title of respect for some notable personage of ancient times: Sir Pandarus of Troy.
a lord or gentleman: noble sirs and ladies.
an ironic or humorous title of respect: sir critic.
Archaic. a title of respect used before a noun to designate profession, rank, etc.: sir priest; sir clerk.

1250–1300; Middle English; unstressed variant of sire Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sir (sɜː)
1.  a formal or polite term of address for a man
2.  archaic a gentleman of high social status
[C13: variant of sire]

Sir (sɜː)
1.  a title of honour placed before the name of a knight or baronet: Sir Walter Raleigh
2.  archaic a title placed before the name of a figure from ancient history

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1300, title of honor of a knight or baronet (until 17c. also a title of priests), variant of sire, originally used only in unstressed position. Generalized as a respectful form of address by mid-14c.; used as a salutation at the beginning of letters from early 15c.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

SIR definition

1. An early system on the IBM 650.
[Listed in CACM 2(5):16, May 1959].
2. Serial Infrared. An infrared standard from IrDA, part of IrDA Data. SIR supports asynchronous communications at 9600 bps - 115.2 Kbps, at a distance of up to 1 metre.

The Free On-line Dictionary of Computing, © Denis Howe 2010
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American Heritage
Abbreviations & Acronyms
shuttle imaging radar
The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences for Sir
After the writing of sir gawain and the green knight, several similar stories followed.
A baronet is styled sir, but a baronetcy is not considered an order of knighthood.
Sir, your eyes and mine are never destined to see that miracle.
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