Sir is an honorary title.

It was once used (without the person's name) as a courtesy title among equals, but is now usually reserved for one of superior rank or stature (e.g. a teacher, a monarch or military officer); as a form of address from a merchant to a customer; in formal correspondence; or to a stranger (Sir, you've dropped your hat.) The equivalent for a woman is madam.

Sir is also the correct styling for a knight or a baronet, used with the knight's given name or full name, but not the with surname alone (Sir Paul McCartney or Sir Paul, not Sir McCartney). The equivalent for a woman is Dame. 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 1968, is often styled Sir Sidney Poitier, particularly in connection with his official ambassadorial duties.

"Sir" derives from the Middle English sire, from the French sieur, meaning "lord," from the Latin adjective senior ("elder"), which yielded titles of respect in many European languages [1].