In common language, the opposite term for sir is madam. If you are using 'Sir' as a title (a knighted person), there are number of paired sex based honorifics for "Sir" : ….
Sir / Madam .
Sir / Lady .
Sir / Dame When using 'sir' as a simple polite title as found on a letter or used to address a stranger: .
sir / madam .
sir / ma'am .
sir / miss However a sex difference does not make the word opposite. Considering that "sir" is an honorific, it positions the person being talked to as superior (in some way) to the talker, the opposite would be a form of address that implies the speaker is superior to the person being talked to. In Shakespearian plays this is handled with the word "sirrah" (e.g. Now, knock when I bid you, sirrah villein! The Taming of the Shrew: I, ii ). This quote also introduces another potential opposite for "sir" which is "villein". This word means a person of low estate who is morally disreputable or who is at the low end of the social scale (a peasant). ( Full Answer )