Definitions from Wiktionary (SIR)

▸ noun:  A man of a higher rank or position.
▸ noun:  A respectful term of address to a man of higher rank or position, particularly:
▸ noun:  to a knight or other low member of the peerage.
▸ noun:  to a superior military officer.
▸ noun:  to a teacher.
▸ noun:  A respectful term of address to an adult male (often older), especially if his name or proper title is unknown.
▸ verb:  (transitive, informal) To address another individual using "sir".
▸ noun:  (British) The titular prefix given to a knight or baronet.
▸ noun:  (Philippines, colloquial or informal) A respectful term of address or reference to a man of higher rank or position before the man's given name or nickname.
▸ noun:  Alternative letter-case form of sir. [A man of a higher rank or position.]
▸ noun:  Alternative form of Syr [(uncommon) Synonym of Syr Darya]
▸ noun:  (manufacturing) Initialism of surface insulation resistance.
▸ noun:  (biochemistry, genetics) Abbreviation of silent information regulator.
▸ adjective:  (epidemiology) Initialism of susceptible-infected/infectious-removed/recovered.

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