Sir (noun)

1. A title used before a man's given name, especially as a term of address in formal situations.
2. A title used for a knight or baronet.

Origin:

Middle english, from old english sīr, from proto-germanic *sīzaz.

Examples:

1. Good morning, sir. How can I help you today?
2. The knight was known as Sir Lancelot.
3. The actor was addressed as Sir Lawrence Olivier.
4. Sir, may I have your attention, please?
5. The baronet was referred to as Sir John.