The Collaborative International Dictionary of English v.0.48:

Sir \Sir\, n. [OE. sire, F. sire, contr. from the nominative L.
   senior an elder, elderly person, compar. of senex,senis, an
   aged person; akin to Gr. ??? old, Skr. sana, Goth. sineigs
   old, sinista eldest, Ir. & Gael. sean old, W. hen. Cf.
   Seignior, Senate, Seneschal, Senior, Senor,
   Signor, Sire, Sirrah.]
   1. A man of social authority and dignity; a lord; a master; a
      gentleman; -- in this sense usually spelled sire. [Obs.]
      [1913 Webster]

            He was crowned lord and sire.         --Gower.
      [1913 Webster]

            In the election of a sir so rare.     --Shak.
      [1913 Webster]

   2. A title prefixed to the Christian name of a knight or a
      baronet.
      [1913 Webster]

            Sir Horace Vere, his brother, was the principal in
            the active part.                      --Bacon.
      [1913 Webster]

   3. An English rendering of the LAtin Dominus, the academical
      title of a bachelor of arts; -- formerly colloquially, and
      sometimes contemptuously, applied to the clergy. --Nares.
      [1913 Webster]

            Instead of a faithful and painful teacher, they hire
            a Sir John, which hath better skill in playing at
            tables, or in keeping of a garden, than in God's
            word.                                 --Latimer.
      [1913 Webster]

   4. 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. "What's that to you, sir?"
      --Sheridan.
      [1913 Webster]

   Note: Anciently, this title, was often used when a person was
         addressed as a man holding a certain office, or
         following a certain business. "Sir man of law." "Sir
         parish priest." --Chaucer.
         [1913 Webster]

   Sir reverance. See under Reverence, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SIR
       Selective Information Retrieval


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SIR
       Sicherheit im Rechenzentrum (TPS)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SIR
       Serial InfraRed (HP)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SIR
       Save Instruction Recognition


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SIR
       Statistical Information Retrieval


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SIR
       Sustained Information Rate (SMDS)


The Free On-line Dictionary of Computing (30 December 2018):

SIR

   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.

   [Reference?]

   (1999-10-14)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

SIRE. A title of honor given to kings or emperors in speaking or writing to
them.