Source : Webster's Revised Unabridged Dictionary (1913)

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.]

            He was crowned lord and sire.         --Gower.

            In the election of a sir so rare.     --Shak.

   2. A title prefixed to the Christian name of a knight or a
      baronet.

            Sir Horace Vere, his brother, was the principal in
            the active part.                      --Bacon.

   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.

            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.

   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.

   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.

   {Sir reverance}. See under {Reverence}, n.

Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)