Sir is a title of respect used in several modern contexts. It was once used (without the person's name) as a courtesy title among equals, but in common usage it is now usually reserved for one of superior rank or stature, such as an educator or commanding officer, or in age (especially by a minor); as a form of address from a merchant to a customer; in formal correspondence (Dear Sir, Right Reverend Sir); or to a stranger (Sir, you've dropped your hat).

The equivalent for a woman is madam.

Origin

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Sir derives from the Middle English sire, from the French sieur (modern French Seignieur), meaning 'lord', from the Latin adjective senior (elder), which yielded titles of respect in many European languages. The form sir entered English in 1297, as title of honor of a knight or baronet, being a variant of Sire, which was already used in English since c.1205 as a title placed before a name and denoting knighthood, and to address the (male) Sovereign since c.1225, with additional general senses of "father, male parent" is from c.1250 and "important elderly man" from 1362.

Formal styling

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In formal protocol Sir is the correct styling for a knight or a baronet (the UK nobiliary rank just below all Peers of the realm), used with the knight's given name or full name, but not with the surname alone (Sir Isaac Newton or Sir Isaac, not Sir Newton). However, in Chinese, the title Sir (爵士) used with the knight's surname or full name. The equivalent for a woman is Dame (for one who holds the title in her own right). The wife of a knight, or baronet, is however styled Lady (Surname).

With regard to British knighthood, a person who is not a subject of the British monarch (i.e. a citizen of a non-Commonwealth country) who receives an honorary knighthood is not entitled to use this style (e.g. Alan Greenspan, KBE, not Sir Alan Greenspan). Dual nationals holding a Commonwealth citizenship that recognises the British monarch as head of state are entitled to use the styling, although common usage varies from country to country: for instance, dual Bahamian-American citizen Sidney Poitier, knighted in 1974, is often styled Sir Sidney Poitier, particularly in connection with his official ambassadorial duties, although he himself rarely employs the title.

Use in disciplined services

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The common use of Sir instead of the rank specific address for a senior officer in a military, police or other hierarchical organisation is rather specific to English. In most languages, no such general address is considered respectful; or the two are combined, as in German Herr followed by the rank. In French the possessive pronoun mon precedes the rank, not unlike My Lord or Monsignor.

When addressing a (male only, unlike in many fictional works) superior (e.g. Officer or Warrant Officer, but--most of the time--not a non-commissioned officer, in the military), "sir" is used to replace his specific rank. However, a United States Marine recruit addresses both commissioned and non-commissoned officers as "sir", especially drill instructors. Enlisted members of the United States Air Force always address superior non-commissioned officers--including Military Training Instructors--as "sir" and, in certain situations, even non-NCOs may be addressed as "sir", most often Senior Airmen (E-4s) serving as training leaders or instructors at technical schools.

Possibly the shortness of the word helps explain another, in a sense compensating, idiomatic but non-official practice in American English: emphatically saying Sir both in front and behind an obedient response in clear voice to the senior, especially during drill, e.g. 'Sir, yes, sir!'

Miscellaneous

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• Until the 17th century it was also a title of priests (the cognate monsignor, from French monseigneur 'my sire', still is used for prelates)
• Various persons in authority, e.g. District Judges in the United Kingdom, are also addressed as "sir", unless they are female, in which case the correct term is "madam".
• Sirrah was a 16th century derivative that implied the inferiority of the addressee.
• The informal forms sirree and siree are merely devised for emphasis in speech, mainly after Yes or No.
• Not to be confused with the now exclusively monarchical (i.e. royal) Sire, even though this has the same etymological root.

Reference

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• EtymologyOnLine

Honorifics | Men's social titles | Knights | British knights | British honours system

Sir | Sir | Sir | Sir | סר | Sir | Sir | Sir | Sir | Сэр | Sir (naziv) | Sir | Sir | சர் | 爵士