Used as a polite form of address for a man, especially in formal or service contexts. Often used in Japanese to address male customers or superiors in a Western-influenced manner.
お客様、サー、お荷物をお持ちしましょうか。
Sir, may I carry your luggage?
Sir Isaac Newton was a physicist.
サー、こちらへどうぞ。
Sir, this way please.