SIR

n. mister, title of respect for a man; term used to formally address a man; gentleman

n. title used before the name of a baronet or knight

The initials SIR may stand for:Sampling Importance ResamplingSerial InfraredSingapore Infantry RegimentSurrey Iron RailwayStaten Island RailwayStatutory Invention RegistrationStandard Information Retrieval a robot from the Invader Zim series.Signal-to-Interference RatioSusceptible–Infected–Recovered modelsSupplemental Inflatable RestraintSecondary Image Registration, a type of passive autofocus system employed mostly in SLR cameras
See more at Wikipedia.org...

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 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).
See more at Wikipedia.org...

Noun
1. a title used before the name of knight or baronet
(hypernym) male aristocrat
(classification) United Kingdom, UK, Great Britain, GB, Britain, United Kingdom of Great Britain and Northern Ireland

Noun
1. term of address for a man
(hypernym) man, adult male

n. sir, mister, title of respect for a man

n. sir