Sirrah was a 16th century derivative that implied the inferiority of the addressee.

Not to be confused with the now exclusively monarchical (i.e. royal) Sire, even though this has the same etymological root.

Various persons in authority, e.g. District Judges in the United Kingdom, are also addressed as "sir".

Sir is an English honorific 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 status, 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.

Sir derives from the Middle French honorific title sire (messire gave 'mylord'), from the Old French sieur (itself a contraction of Seigneur 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.

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). 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 citizen of a Commonwealth realm who receives an honorary knighthood is entitled to use any postnominal letters associated with the knighthood, but not the title "Sir". Dual nationals holding a Commonwealth citizenship that recognise 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.

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 Mein Führer. "Sir", on its own, is sometimes used by schoolchildren to address a male teacher. It is common in British tabloid newspaper slang as a shorthand for 'schoolteacher': Sir's sex shame. Usage of "sir" commonly appears in schools in portions of the Southern United States. 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-commissioned 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!" Often, youths playing American Football at the secondary school level and sometimes at the university level address their coaches as "sir." "Sir" is typically used when the players address the coach as a team, e.g., "Yes, sir!" In both the United States Military and British Armed Forces addressing an NCO as "Sir" is incorrect, and the almost universal response to such an address is "Don't call me sir, I work for a living". In the British Army, however, an NCO is referred to as "sir" when an officer is on parade. In the Royal Canadian Mounted Police only commissioned officers are addressed as "sir", NCOs and constables are addressed by their rank.

Until the 17th century it was also a title of priests (the related word monsignor, from French monseigneur is still used for Catholic prelates). In Icelandic, the cognate word séra is used exclusively to address a priest, together with his first name: a priest called Jón Jónsson will be addressed as séra Jón and referred to as presturinn séra Jón Jónsson ("the priest, séra Jón Jónsson").