Sir is an honorific address used as a courtesy title to address a man without using his given or family name in many English speaking cultures. It is often used in formal correspondence (Dear Sir, Right Reverend Sir).
The term is often reserved for use only towards one of superior rank or status, such as an educator or commanding officer, an elder (especially by a minor), or as a form of address from a merchant to a customer.
Equivalent terms of address are "ma'am" or "madam" in most cases, or in the case of a very young woman, girl, or unmarried woman who prefers to be addressed as such, "miss". The equivalent term for a knighted woman is Dame, or "Lady" for the wife of a knight.
Read more about Sir: Origin, Formal Styling, Use in Disciplined Services, Formal Use in Sports, Miscellaneous
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... of James Stephen, the brother of author and critic Sir Leslie Stephen, the uncle of author Virginia Woolf, and cousin of jurist A.V ... He was already acquainted with Sir Henry Maine, six years his senior, and then newly appointed to the Chair of civil law at Cambridge ... theory James Clerk Maxwell and Liberal Party leader Sir William Harcourt ...
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... Oxford Dr Isaac Watts – Dr Watts' Walk, Abney Park Cemetery, Stoke Newington, London Sir Robert Peel – Market Place, Bury Horatio, Viscount Nelson – on Nelson's Column, Trafalgar ... Bewick – Literary Philosophical Society, Newcastle upon Tyne Sir James Knott – as above George O'Brien Wyndham, 3rd Earl of Egremont – St.Mary's, Petworth ...